fix: prune old backup safety-copies and expired admin sessions instead of letting them pile up forever

Every backup restore was leaving a .before-restore-<timestamp> safety
copy of both mbs.db and .env with no cleanup — would accumulate
forever on a panel that restores regularly. Now keeps the 5 most
recent and prunes the rest right after each restore. Verified: 8
fake copies pruned down to exactly the 5 newest, oldest-first.

admin_sessions rows also never got deleted once expired (only ever
filtered out of queries via expires_at>?) — same shape of problem.
Piggybacks on the existing 90s periodic_sync heartbeat in bot.py
instead of adding a new one.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backup.py

@@ -1,3 +1,4 @@
+import glob
 import io
 import json
 import os
@@ -10,12 +11,23 @@ from config import DB_PATH, BASE_DIR
 
 ENV_PATH = os.path.join(BASE_DIR, ".env")
 MAX_RESTORE_SIZE = 200 * 1024 * 1024
+KEEP_SAFETY_COPIES = 5
 
 
 class RestoreError(Exception):
     pass
 
 
+def _prune_old_safety_copies(keep: int = KEEP_SAFETY_COPIES):
+    for base in (DB_PATH, ENV_PATH):
+        copies = sorted(glob.glob(f"{base}.before-restore-*"))
+        for path in copies[:-keep] if keep > 0 else copies:
+            try:
+                os.remove(path)
+            except OSError:
+                pass
+
+
 def create_backup() -> bytes:
     buf = io.BytesIO()
     db_tmp = DB_PATH + ".backup_snapshot.tmp"
@@ -111,4 +123,5 @@ def restore_backup(data: bytes) -> dict:
     os.chmod(tmp_db_path, 0o600)
     os.replace(tmp_db_path, DB_PATH)
 
+    _prune_old_safety_copies()
     return {"restored_env": restored_env, "safety_copy": safety_copy}