feat(telegram): /topic off + help + auth gate + screenshot debounce

Four production-readiness additions to topic mode:

1. /topic off — clean disable path. Flips telegram_dm_topic_mode.enabled
   to 0 and clears telegram_dm_topic_bindings for this chat. Previously
   users had to edit state.db with sqlite3 to turn the feature off.
   Idempotent: calling /topic off when the chat was never enabled
   returns a friendly no-op message.

2. /topic help — inline usage printed in the DM so users don't have to
   visit docs to discover /topic off, /topic <session-id>, etc.

3. Authorization gate. /topic mutates SQLite side tables and flips the
   root DM into a lobby, so the action must be authorized. Now calls
   self._is_user_authorized(source); unauthorized DMs get a refusal
   instead of activation. Defense in depth on top of the gateway's
   existing pre-route auth.

4. BotFather screenshot debounce. A user repeatedly running /topic
   while Threads Settings is still disabled would previously re-upload
   the same screenshot every time. Now rate-limited to one send per
   5 minutes per chat. /topic off resets the counter so re-enabling
   starts fresh.

Command-def args hint updated: /topic [off|help|session-id].

Docs:
- New /topic subcommands table at the top of the multi-session section
- Disable instructions updated to recommend /topic off first, with the
  raw SQL fallback kept for bulk cleanup
- Under-the-hood list extended with the capability-hint debounce and
  the authorization gate

Tests (6 new):
- /topic help returns usage and doesn't create topic tables
- /topic off disables mode AND clears bindings
- /topic off is idempotent when never enabled
- Unauthorized users get refusal, no tables created
- Capability-hint debounce is per-chat
- /topic off resets both lobby and capability debounce counters

All 402 targeted tests pass. Full gateway sweep: 4809/4810
(pre-existing test_teams::test_send_typing unrelated).

hermes_cli/commands.py

@@ -66,7 +66,7 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("new", "Start a new session (fresh session ID + history)", "Session",
                aliases=("reset",), args_hint="[name]"),
     CommandDef("topic", "Enable or inspect Telegram DM topic sessions", "Session",
-               gateway_only=True, args_hint="[session-id]"),
+               gateway_only=True, args_hint="[off|help|session-id]"),
     CommandDef("clear", "Clear screen and start a new session", "Session",
                cli_only=True),
     CommandDef("redraw", "Force a full UI repaint (recovers from terminal drift)", "Session",