fix(config): reject credential and cache paths inside the application directory

Rooted is not the same as safe, and the gap between the two cost a production
host every one of its API keys on 2026-08-09.

MxGateway:Authentication:SqlitePath was set to an absolute path inside the
directory the upgrade procedure renames to Server.bak.*. That passes the
existing rooted check cleanly. The deploy renamed the directory away, the store
went with it, and the gateway created a fresh empty one at the same path — no
error, no log line. No gRPC consumer could authenticate for two days. The deploy
itself was correct: the binaries were the point of the rename and the store was
collateral.

GatewayConfigPathRules gains AddIfUnderContentRoot, applied to the auth store
and the Galaxy snapshot. Both are written by the running process and both are
lost the same way. The rule compares resolved full paths and requires a
directory-separator boundary, so a sibling directory whose name merely starts
with the content root's ("/srv/app-data" against "/srv/app") is not treated as
inside it — on a fail-closed startup rule, that false positive would be a
gateway that refuses to boot on a legitimate path. Case sensitivity follows the
running OS rather than assuming case-insensitivity everywhere, which would
reject /srv/App as under /srv/app on Linux where they are different directories.

The rule is not exempted in Development. An environment-conditional guard is
never exercised where the mistake is made, and what failed in production was a
config that looked fine.

Secrets:SqlitePath is the same defect one layer down: it shipped as a bare
relative "mxgateway-secrets.db", which is how a stray database landed in
src/…Server/ and tripped the repository's tree-hygiene test. It is bound by the
shared ZB.MOM.WW.Secrets package, so appsettings.json now ships no value and the
default is computed from CommonApplicationData in code — the same mechanism
SEC-33 already used for the Galaxy snapshot, ten lines away, for the same reason.
Setting a default for an unset key is deliberately not the same act as
relocating a value someone configured, which these rules still refuse to do.

Note the migration edge this creates: a host relying on the old repo default now
looks somewhere new, finds nothing, and creates an empty store — this bug
re-introduced by its own fix. Deployed hosts are safe because they set the path
explicitly, in appsettings copied forward or in the service environment. The
latter is the more robust of the two, since it cannot be lost by a missed
preserve step.

docs/GatewayConfiguration.md

@@ -91,7 +91,7 @@ Environment variables use the normal .NET double-underscore form. For example,
 | Option | Default | Description |
 |--------|---------|-------------|
 | `MxGateway:Authentication:Mode` | `ApiKey` | Selects public gRPC authentication. Supported values are `ApiKey` and `Disabled`. `Disabled` bypasses API-key verification and is for local development only. |
-| `MxGateway:Authentication:SqlitePath` | derived from `CommonApplicationData` (`C:\ProgramData\MxGateway\gateway-auth.db` on Windows, `/usr/share/MxGateway/gateway-auth.db` or the container equivalent elsewhere) | SQLite database path for API-key records and audit rows when API-key authentication is enabled. The code default is built from `Environment.GetFolderPath(SpecialFolder.CommonApplicationData)` so the credential store never lands in the launch working directory on a non-Windows host. `appsettings.json` no longer ships an explicit value (SEC-33): the removed Windows literal was byte-identical to the Windows code default, and a Windows-absolute literal is **not** rooted on a Unix host, so it would have resolved against the CWD there. Deployed hosts still override the path through the NSSM environment (`MxGateway__Authentication__SqlitePath`). |
+| `MxGateway:Authentication:SqlitePath` | derived from `CommonApplicationData` (`C:\ProgramData\MxGateway\gateway-auth.db` on Windows, `/usr/share/MxGateway/gateway-auth.db` or the container equivalent elsewhere) | SQLite database path for API-key records and audit rows when API-key authentication is enabled. The code default is built from `Environment.GetFolderPath(SpecialFolder.CommonApplicationData)` so the credential store never lands in the launch working directory on a non-Windows host. `appsettings.json` no longer ships an explicit value (SEC-33): the removed Windows literal was byte-identical to the Windows code default, and a Windows-absolute literal is **not** rooted on a Unix host, so it would have resolved against the CWD there. Deployed hosts still override the path through the NSSM environment (`MxGateway__Authentication__SqlitePath`). The validator additionally rejects a path **inside the application content root**, even an absolute one: the upgrade procedure renames that directory to `Server.bak.*`, which takes the credential store with it and silently starts an empty one. That is not hypothetical — it happened on a production host on 2026-08-09 and no gRPC client could authenticate for two days. |
 | `MxGateway:Authentication:PepperSecretName` | `MxGateway:ApiKeyPepper` | Configuration key used to read the HMAC pepper for API-key secret hashing. The dashboard effective configuration redacts this value. |
 | `MxGateway:Authentication:RunMigrationsOnStartup` | `true` | Runs SQLite auth schema migrations at gateway startup when API-key authentication is enabled. |
 
@@ -291,7 +291,7 @@ section (a sibling of `MxGateway`, not nested under it):
 
 | Option | Default | Description |
 |--------|---------|-------------|
-| `Secrets:SqlitePath` | `mxgateway-secrets.db` | Path to the encrypted secrets store, resolved relative to the app content root when not rooted. |
+| `Secrets:SqlitePath` | `<CommonApplicationData>/MxGateway/mxgateway-secrets.db` | Path to the encrypted secrets store. The default is supplied in code when the key is unset (`C:\ProgramData\MxGateway\...` on Windows), not from `appsettings.json` — a store inside the application directory is renamed away by the upgrade procedure, taking the secrets with it. On non-Windows hosts the default location is usually not writable by a normal user, so a local run must set `Secrets__SqlitePath` explicitly. |
 | `Secrets:MasterKey:Source` | `Environment` | Key-encryption-key (KEK) provider. `Environment` reads a base64-encoded 32-byte key from an env var; `Dpapi` uses a machine-bound key file instead (see below). |
 | `Secrets:MasterKey:EnvVarName` | `ZB_SECRETS_MASTER_KEY` | Env var name the `Environment` provider reads the KEK from. |
 
@@ -402,7 +402,7 @@ model requires otherwise.
 | `MxGateway:Galaxy:CommandTimeoutSeconds` | `60` | Per-command SQL timeout for all Galaxy browse RPCs. |
 | `MxGateway:Galaxy:DashboardRefreshIntervalSeconds` | `30` | Interval between background refreshes of the dashboard Galaxy summary cache. SQL is hit at most once per interval regardless of dashboard render rate. |
 | `MxGateway:Galaxy:PersistSnapshot` | `true` | Persists the latest successful Galaxy browse dataset to disk. When `true`, the cache reloads that snapshot at startup so clients can still browse last-known data while the Galaxy database is unreachable. The restored data is served with `Stale` status until a live query confirms it. |
-| `MxGateway:Galaxy:SnapshotCachePath` | derived from `CommonApplicationData` (`C:\ProgramData\MxGateway\galaxy-snapshot.json` on Windows, `/usr/share/MxGateway/galaxy-snapshot.json` or the container equivalent elsewhere) | File path for the persisted Galaxy browse snapshot. Ignored when `PersistSnapshot` is `false`. The snapshot is written atomically (temp file plus rename). `appsettings.json` no longer ships an explicit value (SEC-33): the option is bound by the shared `ZB.MOM.WW.GalaxyRepository` package, so the gateway supplies the `CommonApplicationData`-derived default when the bound value is blank and registers `GalaxyRepositoryOptionsValidator` to enforce that — when `PersistSnapshot` is `true` — the path is non-blank, valid, and **rooted on the host running the gateway** (`Path.IsPathRooted`, current OS). A bare filename or a foreign-platform literal fails startup instead of resolving against the launch working directory (SEC-01, SEC-33). |
+| `MxGateway:Galaxy:SnapshotCachePath` | derived from `CommonApplicationData` (`C:\ProgramData\MxGateway\galaxy-snapshot.json` on Windows, `/usr/share/MxGateway/galaxy-snapshot.json` or the container equivalent elsewhere) | File path for the persisted Galaxy browse snapshot. Ignored when `PersistSnapshot` is `false`. The snapshot is written atomically (temp file plus rename). `appsettings.json` no longer ships an explicit value (SEC-33): the option is bound by the shared `ZB.MOM.WW.GalaxyRepository` package, so the gateway supplies the `CommonApplicationData`-derived default when the bound value is blank and registers `GalaxyRepositoryOptionsValidator` to enforce that — when `PersistSnapshot` is `true` — the path is non-blank, valid, and **rooted on the host running the gateway** (`Path.IsPathRooted`, current OS). A bare filename or a foreign-platform literal fails startup instead of resolving against the launch working directory (SEC-01, SEC-33). The same validator also rejects a path **inside the application content root**, because the upgrade procedure renames that directory away and the cached snapshot would be discarded on every deploy. |
 
 See [Galaxy Repository Browse](./GalaxyRepository.md) for the RPC surface and
 behavior.