feat(db): table-health readout — per-table bloat metrics in admin card

You can't decide what to maintain without seeing what's bloating. Adds a
read-only health panel driven by Postgres' own statistics views.

- services/db_maintenance.py: get_table_health() queries pg_stat_user_tables +
  pg_total_relation_size + pg_database_size — per-table size, live/dead tuples,
  dead-tuple ratio (the bloat signal), and last (auto)vacuum/(auto)analyze.
- routes/admin.py: admin-only GET /api/admin/db-maintenance/health.
- SettingsView.vue: 'Table health' table in the maintenance card, all tables
  sorted by dead tuples, rows >=20% dead-ratio flagged; total DB size shown;
  refreshes after a Run-now so the dead-tuple drop is visible.
- Tests: health row/size shaping + null-timestamp passthrough; route + service
  surface.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scribe/routes/admin.py

@@ -225,6 +225,14 @@ async def get_db_maintenance():
     })
 
 
+@admin_bp.route("/db-maintenance/health", methods=["GET"])
+@admin_required
+async def get_db_maintenance_health():
+    """Read-only per-table bloat/health stats from Postgres + total DB size."""
+    from scribe.services.db_maintenance import get_table_health
+    return jsonify(await get_table_health())
+
+
 @admin_bp.route("/db-maintenance", methods=["PUT"])
 @admin_required
 async def update_db_maintenance():