Fix bugs, add log rotation, and optimize hot paths

- Fix AttributeError crash on empty request body in md5, hasher, textdiff,
  jwtdecoder, timestamp, passwordgen (get_json without silent=True / or {})
- Fix memory exhaustion in ipcalc: replace list(network.hosts()) with direct
  arithmetic — safe for /8 and larger networks
- Fix O(1M) loop in cronexplainer.get_next_runs: rewrite to skip by
  month/day/hour instead of iterating every minute
- Fix connection leak in notes.ensure_table: add try/finally around conn.close
- Fix admin._ensure_tables / notes._ensure_table running DDL on every request:
  guard with module-level flags (_tables_initialized, _table_ready)
- Fix update_website returning 200 when no row matched; delete_website returning
  success when nothing was deleted; add rowcount checks for both
- Add role validation in admin create_user / update_user (_VALID_ROLES guard)
- Add delimiter length guard in csvviewer (csv.reader requires single char)
- Fix loremipsum: wrap int(count) in try/except ValueError → 400 response
- Fix auth/token: use auth_header[7:] instead of fragile .replace()
- Fix app.py: remove duplicate import sys; cache DB liveness check with 30s TTL
  to avoid a new TCP connection on every frontend page load; move api/setup
  path guard before DB check
- Replace FileHandler with RotatingFileHandler (5 MB / 3 backups) in logger;
  fix relative log paths to absolute paths anchored to __file__
- Wrap all DB connections in try/finally conn.close() throughout admin and notes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/auth/token.py

@@ -18,7 +18,7 @@ def verify_token():
         logger.warning("🔐 Invalid Bearer header")
         return None
 
-    token = auth_header.replace("Bearer ", "")
+    token = auth_header[7:]
     try:
         decoded = decode(token, SECRET_KEY, algorithms=["HS256"])
         return decoded