Make thread/channel/message deletion complete (hard delete)

Deleting a thread, channel, or message was a no-op/soft-delete: messages
were only dropped from the plugin outbox (still in hermes' session store,
hence searchable/recoverable) and channels/threads were merely archived.

Now deletion is complete and non-recoverable, with no search trace:

- purge.py (new): hard-delete from hermes' session store (state.db).
  delete_lane wipes a channel's/thread's sessions + messages; deleting a
  messages row also drops it from the FTS5 index via the delete triggers.
  delete_message removes one message, matched by (session, role, exact
  content, closest timestamp) since plugin m_<hex> ids aren't persisted.
- channels.py: delete() hard-deletes the row (and a channel's child
  threads) instead of archiving.
- outbox.py: add delete_lane() (wipe all frames for a lane) and
  message_info() (read a message's final role/text/ts for the match).
- adapter.py: on_channel_delete wipes outbox + session store;
  on_message_delete purges the session-store row per message.
- App: delete confirmations no longer claim history stays for search;
  ChannelStore removes a channel's threads on channel delete.
- Docs updated to describe hard deletion.

docs/protocol/frames.schema.json

@@ -86,7 +86,7 @@
       "search": { "payload": { "query": { "type": "string" }, "scope": { "type": "string", "enum": ["all", "chat"] }, "chat_id": { "type": "string" }, "thread_id": { "type": "string" }, "limit": { "type": "integer", "description": "Optional; server default 20." } } },
       "sync": { "description": "Reconnect catch-up; replays undelivered outbox frames only (not full history).", "payload": { "cursor": { "type": "integer" } } },
       "history": { "description": "Load a page of full message history for a chat/thread (initial open, scroll-up pagination).", "payload": { "before_message_id": { "type": "string", "description": "Return messages older than this (omit for newest page)." }, "limit": { "type": "integer", "description": "Max messages (default 50, max 200)." } } },
-      "message.delete": { "description": "Delete the given message(s) from a chat/thread. The server removes them from the outbox (so history/sync no longer return them) and broadcasts message.deleted to every device. Idempotent: a message already gone (pruned) still yields a message.deleted broadcast.", "payload": { "message_ids": { "type": "array", "items": { "type": "string" }, "description": "One or more message_id values to delete." } } },
+      "message.delete": { "description": "Completely delete the given message(s) from a chat/thread. The server removes them from the outbox (so history/sync no longer return them) and from the hermes session store (so no search trace survives and they are not recoverable), then broadcasts message.deleted to every device. Idempotent: a message already gone (pruned) still yields a message.deleted broadcast.", "payload": { "message_ids": { "type": "array", "items": { "type": "string" }, "description": "One or more message_id values to delete." } } },
       "fcm.register": { "payload": { "fcm_token": { "type": "string" }, "ntfy_topic": { "type": "string" } } },
       "ping": { "payload": { "ts": { "type": "integer" } } }
     }