fix(telegram): polish topic mode — CASCADE, General-topic handling, rename guard, debounce

Five follow-ups to topic mode based on integration audit:

1. ON DELETE CASCADE on telegram_dm_topic_bindings.session_id. Session
   pruning (manual /delete, auto-cleanup, any future prune job) would
   have thrown 'FOREIGN KEY constraint failed' for sessions bound to a
   topic. Migration bumped to v2, rebuilds the bindings table in place
   if FK lacks CASCADE. Idempotent; only runs once per DB.

2. Never auto-rename operator-declared topics. If an operator has
   extra.dm_topics configured AND a user runs /topic, messages in those
   pre-declared topics would previously trigger auto-rename and silently
   mutate operator config. _rename_telegram_topic_for_session_title now
   early-returns when _get_dm_topic_info returns a dict for this
   (chat_id, thread_id). Uses class-based lookup (not hasattr) so
   MagicMock test fixtures don't accidentally trip the guard.

3. General topic handling. Telegram's General (pinned top) topic in a
   forum-enabled private chat may send messages with message_thread_id=1
   or omit thread_id entirely depending on client. Both are now treated
   as the root lobby, not a topic lane. Prevents users from
   accidentally burning a session on the General topic.

4. Debounce the root-lobby reminder. 30-second cooldown per chat so a
   user who forgets topic mode is enabled and types ten messages in the
   root gets one reminder, not ten. Explicit command replies
   (/new-in-lobby, /topic <session-id>) still land every time.

5. Docs: added under-the-hood invariants for the above, plus a
   Downgrade section explaining that rolling back to a pre-/topic
   Hermes build leaves the DB tables orphaned but harmless — DMs just
   revert to native per-thread isolation.

Tests:
- test_operator_declared_topic_is_not_auto_renamed
- test_general_topic_is_treated_as_root_lobby
- test_lobby_reminder_is_debounced_per_chat
- test_binding_survives_session_deletion_via_cascade
- test_migration_rebuilds_v1_binding_table_with_cascade_fk

Validated: 4803/4804 tests pass (tests/gateway/ + tests/test_hermes_state.py).
Sole failure is a pre-existing test_teams::test_send_typing flake
unrelated to this PR.

hermes_state.py

@@ -2155,6 +2155,11 @@ class SessionDB:
         reconciliation. Operators must be able to upgrade Hermes, keep the old
         Telegram bot behavior running, and only mutate topic-mode state when the
         user executes /topic to opt into the feature.
+
+        Schema versions:
+          v1 — initial shape (no ON DELETE CASCADE on session_id FK)
+          v2 — session_id FK gets ON DELETE CASCADE so session pruning
+               automatically clears bindings.
         """
         def _do(conn):
             conn.executescript(
@@ -2177,7 +2182,7 @@ class SessionDB:
                     thread_id TEXT NOT NULL,
                     user_id TEXT NOT NULL,
                     session_key TEXT NOT NULL,
-                    session_id TEXT NOT NULL REFERENCES sessions(id),
+                    session_id TEXT NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
                     managed_mode TEXT NOT NULL DEFAULT 'auto',
                     linked_at REAL NOT NULL,
                     updated_at REAL NOT NULL,
@@ -2191,10 +2196,55 @@ class SessionDB:
                 ON telegram_dm_topic_bindings(user_id, chat_id);
                 """
             )
+
+            # v1 → v2: rebuild telegram_dm_topic_bindings if its session_id FK
+            # lacks ON DELETE CASCADE. SQLite can't ALTER a foreign key, so we
+            # rebuild the table. Only runs once per DB (version gate).
+            current = conn.execute(
+                "SELECT value FROM state_meta WHERE key = ?",
+                ("telegram_dm_topic_schema_version",),
+            ).fetchone()
+            current_version = int(current[0]) if current and str(current[0]).isdigit() else 0
+            if current_version < 2:
+                fk_rows = conn.execute(
+                    "PRAGMA foreign_key_list('telegram_dm_topic_bindings')"
+                ).fetchall()
+                needs_rebuild = any(
+                    row[2] == "sessions" and (row[6] or "") != "CASCADE"
+                    for row in fk_rows
+                )
+                if needs_rebuild:
+                    conn.executescript(
+                        """
+                        CREATE TABLE telegram_dm_topic_bindings_new (
+                            chat_id TEXT NOT NULL,
+                            thread_id TEXT NOT NULL,
+                            user_id TEXT NOT NULL,
+                            session_key TEXT NOT NULL,
+                            session_id TEXT NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
+                            managed_mode TEXT NOT NULL DEFAULT 'auto',
+                            linked_at REAL NOT NULL,
+                            updated_at REAL NOT NULL,
+                            PRIMARY KEY (chat_id, thread_id)
+                        );
+                        INSERT INTO telegram_dm_topic_bindings_new
+                            SELECT chat_id, thread_id, user_id, session_key,
+                                   session_id, managed_mode, linked_at, updated_at
+                            FROM telegram_dm_topic_bindings;
+                        DROP TABLE telegram_dm_topic_bindings;
+                        ALTER TABLE telegram_dm_topic_bindings_new
+                            RENAME TO telegram_dm_topic_bindings;
+                        CREATE UNIQUE INDEX idx_telegram_dm_topic_bindings_session
+                            ON telegram_dm_topic_bindings(session_id);
+                        CREATE INDEX idx_telegram_dm_topic_bindings_user
+                            ON telegram_dm_topic_bindings(user_id, chat_id);
+                        """
+                    )
+
             conn.execute(
                 "INSERT INTO state_meta (key, value) VALUES (?, ?) "
                 "ON CONFLICT(key) DO UPDATE SET value = excluded.value",
-                ("telegram_dm_topic_schema_version", "1"),
+                ("telegram_dm_topic_schema_version", "2"),
             )
         self._execute_write(_do)