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.Host.Tests/StartupValidatorTests.cs

@@ -232,15 +232,22 @@ public class StartupValidatorTests
         Assert.Null(ex);
     }
 
+    /// <summary>
+    /// The inverse of the rule this replaces. SiteDbPath was mandatory for Site
+    /// nodes until LocalDb Phase 2 moved the site tables into the consolidated
+    /// LocalDb database; it now names only the legacy file the boot-time migrator
+    /// drains, so its absence means "nothing to migrate". Requiring it would make
+    /// every already-migrated node carry a dead key forever.
+    /// </summary>
     [Fact]
-    public void Site_MissingSiteDbPath_FailsValidation()
+    public void Site_MissingSiteDbPath_IsAccepted_BecauseThePathIsMigrationOnly()
     {
         var values = ValidSiteConfig();
         values.Remove("ScadaBridge:Database:SiteDbPath");
         var config = BuildConfig(values);
 
-        var ex = Assert.Throws<InvalidOperationException>(() => StartupValidator.Validate(config));
-        Assert.Contains("SiteDbPath required for Site nodes", ex.Message);
+        var ex = Record.Exception(() => StartupValidator.Validate(config));
+        Assert.Null(ex);
     }
 
     [Fact]