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/util/logger.py

@@ -1,21 +1,39 @@
 import logging
 import os
+from logging.handlers import RotatingFileHandler
 
-os.makedirs("logs", exist_ok=True)
+_LOG_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), "..", "logs")
+os.makedirs(_LOG_DIR, exist_ok=True)
 
-fmt = "%(asctime)s [%(levelname)s] %(message)s"
+_FMT = "%(asctime)s [%(levelname)s] %(message)s"
+_formatter = logging.Formatter(_FMT)
 
-error_handler = logging.FileHandler("logs/error.log")
-error_handler.setLevel(logging.ERROR)
+_MAX_BYTES = 5 * 1024 * 1024  # 5 MB per file
+_BACKUP_COUNT = 3
+
+
+def _rotating(filename, level):
+    h = RotatingFileHandler(
+        os.path.join(_LOG_DIR, filename),
+        maxBytes=_MAX_BYTES,
+        backupCount=_BACKUP_COUNT,
+        encoding="utf-8",
+    )
+    h.setLevel(level)
+    h.setFormatter(_formatter)
+    return h
+
+
+_console = logging.StreamHandler()
+_console.setFormatter(_formatter)
 
 logging.basicConfig(
     level=logging.INFO,
-    format=fmt,
     handlers=[
-        logging.FileHandler("logs/app.log"),
-        error_handler,
-        logging.StreamHandler()
-    ]
+        _rotating("app.log", logging.INFO),
+        _rotating("error.log", logging.ERROR),
+        _console,
+    ],
 )
 
 logger = logging.getLogger("main")