feat(external-systems): deploy per-system retry policy to sites + CLI/management surface (register row 27)

An external system's MaxRetries/RetryDelay existed on the central entity but never
reached a site and had no management surface, so every ExternalSystem.CachedCall
buffered with the S&F engine default (DefaultMaxRetries 50 x DefaultRetryInterval
30s ~= 25 min to park) whatever central had configured. The sibling
DatabaseConnectionArtifact had always carried its equivalents.

The consumption end was already wired: ExternalSystemClient.CachedCallAsync already
read system.MaxRetries/RetryDelay and passed them to EnqueueAsync with the 0-as-unset
convention. Only the transport and persistence were missing.

Commons
- ExternalSystemArtifact gains MaxRetries/RetryDelay as optional positional params
  (0 / TimeSpan.Zero = unset), matching DatabaseConnectionArtifact rather than going
  nullable, so it maps 1:1 onto the entity and the proto3 scalars.
- Create/UpdateExternalSystemCommand gain nullable MaxRetries/RetryDelay, following
  the existing TimeoutSeconds "omit to preserve" convention. Both additive.

Central
- ManagementActor create maps both; update preserves-if-null.
- ArtifactDeploymentService projects them onto the artifact.

Wire
- site_command.proto ExternalSystemArtifactDto gains max_retries=7 /
  retry_delay=8 (google.protobuf.Duration), mirroring DatabaseConnectionArtifactDto.
  Additive: a pre-existing site decodes them as 0/absent, which IS the unset value,
  so old behaviour is preserved. Vendored C# regenerated via docker/regen-proto.sh
  sitecommand; csproj Protobuf item re-commented, no message types added or removed.

Site
- external_systems gains max_retries / retry_delay_ms via AddColumnIfMissing,
  DEFAULT 0 = unset. Storage upsert, repository SELECT/mapping (new ordinals 6/7
  appended so existing ones are untouched), apply path, and the legacy migrator's
  explicit column list all carry them.

Replicated-table decision: external_systems IS in SiteLocalDbSetup.ReplicatedTables,
so this was checked against the AckTime/native_alarm_state precedent. Real columns
are correct here, not a JSON bag: the table has no general-purpose config column
(auth_configuration and method_definitions are both semantically specific),
timeout_seconds already set the additive-column precedent on this exact table, and
sibling database_connections stores the same two values as plain columns. Verified in
LocalDb source that RegisterReplicated re-reads PRAGMA table_info every boot and its
install script is DROP-then-CREATE, so capture triggers rebuild over the new column
set automatically, and SiteStorageSchema.Apply runs before registration on the same
boot. The only cost is the ReplicatedTable.ComputeDigest change, which the sync
handshake compares fail-closed - a mixed-version pair refuses to sync loudly instead
of diverging silently. Both-nodes-together is already the standing site-pair rule, so
no new operational constraint; recorded in topology-guide.md.

Also fixed: ArtifactDiff.CompareExternalSystem never compared MaxRetries/RetryDelay,
so a bundle import that changed a system's retry budget previewed as "Identical" -
harmless while the values were inert, a live behaviour change now.

No EF migration: the central ExternalSystemDefinitions columns already exist
(InitialSchema). Central UI already had both form fields.

Tests: site storage round-trip + unset-default, site apply path, central artifact
projection, proto round-trip (via the shared sample), management create/preserve,
CLI option parse + alias, and diff reporting. Fail-before verified on the two core
tests (expected 7 got 0 on the read path; expected 9 got 0 on the apply path).

Docs: Component-ExternalSystemGateway, Component-StoreAndForward, Component-CLI,
components/StoreAndForward, components/SiteRuntime, deployment/topology-guide,
CLI README (which was also missing --timeout-seconds entirely).

docs/requirements/Component-StoreAndForward.md

@@ -63,6 +63,19 @@ For the external-system-call and cached-database-write categories, retry setting
 - **External systems**: Each external system definition includes max retry count and time between retries.
 - **Cached database writes**: Each database connection definition includes max retry count and time between retries.
 
+Both are carried to the site in the deployment artifact and read back out of the site's own
+tables by the gateway, which passes them to `EnqueueAsync`. An **unset** source-entity value
+(`MaxRetries` `0` / `RetryDelay` `TimeSpan.Zero`) is passed as `null`, so the engine's
+`DefaultMaxRetries` (50) and `DefaultRetryInterval` (30 s) apply — roughly 25 minutes to park.
+The translation is deliberate and must not be removed: the engine reads a stored `MaxRetries`
+of `0` as **no limit**, so forwarding an unconfigured entity's `0` verbatim would turn every
+cached call into an unbounded retry loop.
+
+> Until deferred-work row 27 was fixed, this was true of database connections only.
+> `ExternalSystemArtifact` carried no retry fields and the site `external_systems` table had no
+> columns for them, so an external system's centrally-configured policy never reached the site
+> and every external-system cached call silently used the 50 x 30 s default.
+
 The **notification** category retries differently: it has no source-entity setting. The site→central forward uses a single fixed retry interval configured in the host `appsettings.json`. This interval is infrastructure config for reaching the central cluster, not a per-notification-list setting. It applies uniformly to every buffered notification regardless of its target list. A buffered notification is retried at that interval until central acks it; the engine's `DefaultMaxRetries` cap still applies (matching the cached-call categories) and a notification whose retries are exhausted under a sustained central outage parks like any other buffered message. The cap is sized so the normal central-recovery window stays well inside it — central, once reachable, owns delivery, retry, and parking from the ack point on.
 
 The retry interval is **fixed** (not exponential backoff). Fixed interval is sufficient for the expected use cases.