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.StoreAndForward/StoreAndForwardOptions.cs

@@ -5,12 +5,15 @@ namespace ZB.MOM.WW.ScadaBridge.StoreAndForward;
 /// </summary>
 public class StoreAndForwardOptions
 {
-    /// <summary>Path to the SQLite database for S&amp;F message persistence.</summary>
+    /// <summary>
+    /// Path to the legacy standalone store-and-forward SQLite file. <b>Migration-only.</b>
+    /// The buffer itself lives in the consolidated LocalDb database (<c>LocalDb:Path</c>)
+    /// as of LocalDb Phase 2; this path is read once at boot by
+    /// <c>SiteLocalDbLegacyMigrator</c> to drain a pre-Phase-2 file, and is otherwise
+    /// unused. A node that has already migrated may leave it set or unset.
+    /// </summary>
     public string SqliteDbPath { get; set; } = "./data/store-and-forward.db";
 
-    /// <summary>Whether to replicate buffer operations to standby node.</summary>
-    public bool ReplicationEnabled { get; set; } = true;
-
     /// <summary>Default retry interval for messages without per-source settings.</summary>
     public TimeSpan DefaultRetryInterval { get; set; } = TimeSpan.FromSeconds(30);