feat(db): backfill, DB-backed web scans, firewall snapshot view (R001/S001/F001)

- R001: app/db/backfill.py imports git-history blocklist -> ledger, firewall
  snapshots, and per-source requests.csv (idempotent; ran live migrating 705
  historical blocklist entries).
- S001: web_scan.latest_scans() now merges SQLite 'scans' records so the
  dashboard surfaces CLI/restart scans (_db_scans).
- F001: /api/firewall/snapshots + 'Snapshot history' panel; added
  recent_firewall_snapshots() query with entry counts.

app/web/routes.py

@@ -265,6 +265,19 @@ def register_routes(app: Flask,
             "snapshot_dir": MIKROTIK_DATA_DIR,
         })
 
+    @app.get("/api/firewall/snapshots")
+    def firewall_snapshots():
+        """Recent firewall snapshots persisted in the SQLite store (F001)."""
+        try:
+            from ..db import get_connection
+            from ..db import repositories as repo
+            conn = get_connection()
+            limit = request.args.get("limit", 10, type=int)
+            return jsonify({"snapshots": repo.recent_firewall_snapshots(
+                conn, limit=limit)})
+        except Exception as exc:
+            return jsonify({"error": str(exc), "snapshots": []}), 503
+
     @app.post("/api/firewall/sync")
     def firewall_sync():
         return _sync_firewall(dry_run=True)