Task #209 exit gate — seed-creds fix + live Modbus verification (4/5 stages)

Booted the server against the Modbus seed end-to-end to exercise the
factory wiring shipped in #216 + #217. Surfaced two real issues with
the seeds themselves; fixed both:

1. **Missing ClusterNodeCredential.** `sp_GetCurrentGenerationForCluster`
   enforces `ClusterNodeCredential.Value = SUSER_SNAME()` and aborts
   with `RAISERROR('Unauthorized: caller sa is not bound to NodeId')`.
   All four seed scripts now insert the binding row alongside the
   ClusterNode row. Without this, the server fails bootstrap with
   `BootstrapException: Central DB unreachable and no local cache
   available` (the Unauthorized error gets swallowed on top of the
   HTTP fallback path).

2. **Config cache gitignore.** Running the server from the repo root
   writes `config_cache.db` + `config_cache-log.db` next to the cwd,
   outside the existing `src/.../Server/config_cache.db` pattern. Add
   a `config_cache*.db` pattern so any future run location is covered.

## Verified live against Modbus

Booted server against `seed-modbus-smoke.sql` → pymodbus standard
fixture → ran `scripts/e2e/test-modbus.ps1 -BridgeNodeId "ns=2;s=HR200"`:

    === Modbus e2e summary: 4/5 passed ===
    [PASS] Probe
    [PASS] Driver loopback
    [PASS] Server bridge (driver → server → client)
    [FAIL] OPC UA write bridge (0x801F0000)
    [PASS] Subscribe sees change

The forward direction + subscription delivery are proven working through
the server. The reverse-write failure is a seed-or-ACL issue — server
log shows no exception on the write path, so the client-side status is
coming from the stack's type/ACL guards. Tracking as a follow-up issue
so the remaining three factory wirings can be smoke-booted against the
same pattern.

Note for future runs: two stale v1 `ZB.MOM.WW.LmxOpcUa.Host.exe`
processes from `C:\publish\lmxopcua\instance{1,2}\` squat on ports
4840 + 4841 on this dev box; kill them first or bump the seed's
DashboardPort.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/smoke/seed-modbus-smoke.sql

@@ -65,6 +65,14 @@ INSERT dbo.ClusterNode(NodeId, ClusterId, RedundancyRole, Host, OpcUaPort, Dashb
 VALUES (@NodeId, @ClusterId, 'Primary', 'localhost', 4840, 5000,
         'urn:OtOpcUa:modbus-smoke-node', 200, 1, 'modbus-smoke');
 
+-- Bind the SQL login this smoke test connects as to the node identity. The
+-- sp_GetCurrentGenerationForCluster + sp_UpdateClusterNodeGenerationState
+-- sprocs raise RAISERROR('Unauthorized: caller %s is not bound to NodeId %s')
+-- when this row is missing. `Kind='SqlLogin'` / `Value='sa'` matches the
+-- container's SA user; rotate Value for real deployments using a non-SA login.
+INSERT dbo.ClusterNodeCredential(NodeId, Kind, Value, Enabled, CreatedBy)
+VALUES (@NodeId, 'SqlLogin', 'sa', 1, 'modbus-smoke');
+
 -- 2. Draft generation.
 DECLARE @Gen bigint;
 INSERT dbo.ConfigGeneration(ClusterId, Status, CreatedBy)