fix(server): close residual review findings F-5..F-8

- Continuous endpoints reject non-list channels (400), null no longer wipes stored list (F-5)
- '@'-prefixed legacy channel values normalized on load/save in manager and store (F-6)
- _run_loop guarded: unexpected exceptions logged, backoff retry, no silent thread death (F-7)
- +8 tests: continuous validation, normalization, loop survival, join-timeout/tombstone (F-8)
- 58 tests passing; REVIEW.md updated

app_state.py

@@ -158,7 +158,7 @@ class StateStore:
                 "enabled": bool(config.get("enabled", True)),
                 "interval_minutes": max(1, int(config.get("interval_minutes", 1) or 1)),
                 "channels": [
-                    str(item).strip()
+                    str(item).strip().lstrip("@")
                     for item in config.get("channels", [])
                     if str(item).strip()
                 ],