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/appsettings.Site.json

@@ -23,6 +23,10 @@
       "MinNrOfMembers": 1
     },
     "Database": {
+      // Migration-only as of LocalDb Phase 2. The site config tables now live in the
+      // consolidated LocalDb database (LocalDb:Path). SiteDbPath is read once at boot to drain
+      // a pre-Phase-2 scadabridge.db, and is unused after that - keep it until this node has
+      // started once.
       "SiteDbPath": "./data/scadabridge.db"
     },
     "DataConnection": {
@@ -31,8 +35,11 @@
       "WriteTimeout": "00:00:30"
     },
     "StoreAndForward": {
-      "SqliteDbPath": "./data/store-and-forward.db",
-      "ReplicationEnabled": true
+      // Migration-only as of LocalDb Phase 2. The store-and-forward buffer now lives in the
+      // consolidated LocalDb database (LocalDb:Path) as the replicated sf_messages table.
+      // SqliteDbPath is read once at boot by SiteLocalDbLegacyMigrator to drain a pre-Phase-2
+      // file, and is unused after that - keep it until this node has started once.
+      "SqliteDbPath": "./data/store-and-forward.db"
     },
     "Communication": {
       "_centralContactPoints": "Host-016: each entry MUST be a central node's remoting endpoint, NOT this site's own remoting port. The single dev-loopback default below points only at central-a (localhost:8081). In a multi-central deployment add the second central node here (e.g. 'akka.tcp://scadabridge@central-b-host:8081') so ClusterClient can fail over when central-a is down. The previous template listed localhost:8082 as the second contact — that is THIS site's own RemotingPort and is a permanent failure in the initial-contact rotation.",