chore(config): retire ReplicationEnabled, make legacy db paths migration-only

LocalDb Phase 2 deleted the bespoke replicators, so three config keys changed
meaning or died outright:

- ScadaBridge:StoreAndForward:ReplicationEnabled is fully dead. Deleted the
  property, its 10 config entries, and the 5 test references.
- SqliteDbPath / SiteDbPath are now migration-only: they name the legacy files
  SiteLocalDbLegacyMigrator drains at boot, not live databases. Both mandatory
  rules are relaxed accordingly (StartupValidator's Site-only Require, and the
  S&F validator's non-empty rule) — an absent value now means "nothing to
  migrate", so an already-migrated node can drop the key. DatabaseOptions-
  Validator still rejects a present-but-blank value.
- SiteRuntime:ConfigFetchRetryCount's only reader was SiteReplicationActor.
  Deleted with its validator rule.

The two path keys stay present in every config, now with a comment explaining
why: removing them would strand un-migrated data on a node that has not yet
started once.

Both relaxations are pinned by the inverse of the test they replace
(Site_MissingSiteDbPath_IsAccepted..., EmptySqliteDbPath_IsAccepted...), each
verified to fail with the old rule restored.

Note: deploy/wonder-app-vd03/appsettings.Site.json is under a gitignored
deploy/ tree, so its edit is local-only and must be repeated on the box.

Claude-Session: https://claude.ai/code/session_01BL2Vu1ESDQ9SCN4gVKkdts

src/ZB.MOM.WW.ScadaBridge.Host/StartupValidator.cs

@@ -94,7 +94,7 @@ public static class StartupValidator
                 _ => seedNodes != null && seedNodes.Count >= 2,
                 "must have at least 2 entries")
             // The big Site-only block: GrpcPort/MetricsPort validity + cross-field
-            // collisions + SiteDbPath + seed-node-port loop, in the original order.
+            // collisions + seed-node-port loop, in the original order.
             .When(role == "Site", p =>
             {
                 // GrpcPort range, then GrpcPort vs RemotingPort.
@@ -110,9 +110,12 @@ public static class StartupValidator
                 p.Require("ScadaBridge:Node:MetricsPort", _ => metricsPort != port, "must differ from RemotingPort");
                 p.Require("ScadaBridge:Node:MetricsPort", _ => metricsPort != grpcPort, "must differ from GrpcPort");
 
-                p.Require("ScadaBridge:Database:SiteDbPath",
-                    _ => !string.IsNullOrEmpty(configuration.GetSection("ScadaBridge:Database")["SiteDbPath"]),
-                    "required for Site nodes");
+                // ScadaBridge:Database:SiteDbPath was required here until LocalDb
+                // Phase 2. The site's tables now live in the consolidated LocalDb
+                // database (LocalDb:Path, which SiteServiceRegistration requires),
+                // and SiteDbPath survives only as the legacy migration source — so
+                // its absence means "nothing to migrate", not a misconfiguration.
+                // DatabaseOptionsValidator still rejects a present-but-blank value.
 
                 // A seed node must reference an Akka.Remote endpoint, never the
                 // Kestrel HTTP/2 gRPC port. A seed entry whose port equals this node's