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

tests/ZB.MOM.WW.ScadaBridge.StoreAndForward.Tests/StoreAndForwardOptionsValidatorTests.cs

@@ -39,13 +39,20 @@ public class StoreAndForwardOptionsValidatorTests
         Assert.Contains("RetryTimerInterval", result.FailureMessage);
     }
 
+    /// <summary>
+    /// The inverse of the rule this replaces. Before LocalDb Phase 2 an empty
+    /// SqliteDbPath was rejected, because it was the live buffer file and an empty
+    /// value failed opaquely at first enqueue. The buffer now lives in the
+    /// consolidated LocalDb database, so the key is only the legacy migration
+    /// source and an empty value legitimately means "nothing to migrate" — a node
+    /// that has already migrated must be able to drop it.
+    /// </summary>
     [Fact]
-    public void EmptySqliteDbPath_IsRejected()
+    public void EmptySqliteDbPath_IsAccepted_BecauseThePathIsMigrationOnly()
     {
         var result = Validate(new StoreAndForwardOptions { SqliteDbPath = "" });
 
-        Assert.True(result.Failed);
-        Assert.Contains("SqliteDbPath", result.FailureMessage);
+        Assert.False(result.Failed);
     }
 
     // ── R2 T9: sweep-tuning eager validation (N4) ──