Fix all activity system edge cases

Critical fixes:
- Add threading.Lock for all shared mutable state (override, cache, current activity)
- Atomic YAML writes (temp file + os.replace) to prevent corruption on crash
- Deep-copy cache on reads to prevent callers from mutating shared state

High-severity fixes:
- Validate entries in pick_activity_for_mood() — skip/log malformed instead of KeyError
- Log warning on unrecognized activity type fallback
- Normalize empty-string state to None (avoid 'None' display)
- release_manual_override() now uses force=True so bot always shows activity
- Add try/except in release_manual_override() to handle failures gracefully

Medium fixes:
- Remove dead 'test' mood from activities.yaml
- Validate name length (128 char Discord limit) in CRUD and manual set
- Validate streaming entries have URL in CRUD path
- Add JSON parse error handling in API routes
- on_ready preserves active manual override instead of overwriting
- Log override expiry timestamp (HH:MM:SS) for easier debugging
- exc_info=True on presence update errors for full stack traces

Low fixes:
- JS activitySetFromEntry() shows notification on parse error

bot/activities.yaml

@@ -505,10 +505,6 @@ normal:
     name: Gintama
     weight: 1
     state: Comedy Anime
-  test:
-  - type: playing
-    name: G
-    weight: 2
 evil:
   aggressive:
   - type: listening