feat: admin notification system with priority ordering, bell icon, and /notifications page

Two-tier notification system: new_booking (all public bookings) + pending_booking (notes/today).
Priority-sorted queue, unread count polling, enriched responses with user_name/booking_start_time.
Fix critical bug: edit_requested cleanup was broken (wrong reason string in 3 handlers).
Add 15 new tests covering priority ordering, enrichment, and notification creation flows.
Update Admin Manual, Technical Manual, and gap backlog docs.

obsidian/.obsidian/workspace.json

@@ -171,11 +171,11 @@
   },
   "active": "0e456d61bc5b6ded",
   "lastOpenFiles": [
+    "Crussell/Overview.md",
+    "Crussell/Future Work - Gap Backlog.md",
     "Crussell/Technical Manual.md",
     "Crussell/User Manual.md",
     "Crussell/Admin Manual.md",
-    "Crussell/Overview.md",
-    "Crussell/Future Work - Gap Backlog.md",
     "Crussell/Test Implementation Plan.md",
     "Crussell/Crussell Nails.md",
     "Crussell/Backend/bookings.md",