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/tools/cronexplainer.py

@@ -96,27 +96,97 @@ def build_summary(minute, hour, day, month, weekday):
 
 
 def get_next_runs(minute_vals, hour_vals, day_vals, month_vals, weekday_vals):
-    # cron weekday 0=Sun..6=Sat,7=Sun -> python weekday 0=Mon..6=Sun
+    """Return up to 5 upcoming run times.
+
+    Uses targeted jumps (skip month, day, hour) instead of iterating every
+    minute, so performance is acceptable even for rare expressions like
+    '0 0 29 2 *' (Feb 29 at midnight).
+    """
+    # cron weekday 0=Sun..6=Sat,7=Sun  →  Python weekday 0=Mon..6=Sun
     py_weekdays = set((wd + 6) % 7 for wd in weekday_vals)
+
     minute_set = set(minute_vals)
     hour_set = set(hour_vals)
     day_set = set(day_vals)
     month_set = set(month_vals)
 
+    minute_list = sorted(minute_vals)
+    hour_list = sorted(hour_vals)
+    month_list = sorted(month_vals)
+
+    if not minute_list or not hour_list or not month_list:
+        return []
+
     now = datetime.now().replace(second=0, microsecond=0) + timedelta(minutes=1)
+    max_date = now + timedelta(days=4 * 366)
     results = []
     current = now
 
-    for _ in range(2 * 366 * 24 * 60):
-        if len(results) >= 5:
-            break
-        if (current.month in month_set and
-                current.day in day_set and
-                current.weekday() in py_weekdays and
-                current.hour in hour_set and
-                current.minute in minute_set):
-            results.append(current.isoformat())
-        current += timedelta(minutes=1)
+    def _advance_from_results():
+        """Move current to the next candidate after a match."""
+        nonlocal current
+        next_mins = [m for m in minute_list if m > current.minute]
+        if next_mins:
+            current = current.replace(minute=next_mins[0])
+            return
+        next_hours = [h for h in hour_list if h > current.hour]
+        if next_hours:
+            current = current.replace(hour=next_hours[0], minute=minute_list[0])
+            return
+        current = (current + timedelta(days=1)).replace(hour=hour_list[0], minute=minute_list[0])
+
+    while current <= max_date and len(results) < 5:
+        # ── month ──────────────────────────────────────────────────────────
+        if current.month not in month_set:
+            next_months = [m for m in month_list if m > current.month]
+            if next_months:
+                current = current.replace(
+                    month=next_months[0], day=1,
+                    hour=hour_list[0], minute=minute_list[0]
+                )
+            else:
+                current = current.replace(
+                    year=current.year + 1, month=month_list[0], day=1,
+                    hour=hour_list[0], minute=minute_list[0]
+                )
+            continue
+
+        # ── day + weekday ──────────────────────────────────────────────────
+        if current.day not in day_set or current.weekday() not in py_weekdays:
+            current = (current + timedelta(days=1)).replace(
+                hour=hour_list[0], minute=minute_list[0]
+            )
+            continue
+
+        # ── hour ───────────────────────────────────────────────────────────
+        if current.hour not in hour_set:
+            next_hours = [h for h in hour_list if h > current.hour]
+            if next_hours:
+                current = current.replace(hour=next_hours[0], minute=minute_list[0])
+            else:
+                current = (current + timedelta(days=1)).replace(
+                    hour=hour_list[0], minute=minute_list[0]
+                )
+            continue
+
+        # ── minute ─────────────────────────────────────────────────────────
+        if current.minute not in minute_set:
+            next_mins = [m for m in minute_list if m > current.minute]
+            if next_mins:
+                current = current.replace(minute=next_mins[0])
+            else:
+                next_hours = [h for h in hour_list if h > current.hour]
+                if next_hours:
+                    current = current.replace(hour=next_hours[0], minute=minute_list[0])
+                else:
+                    current = (current + timedelta(days=1)).replace(
+                        hour=hour_list[0], minute=minute_list[0]
+                    )
+            continue
+
+        # ── match ──────────────────────────────────────────────────────────
+        results.append(current.isoformat())
+        _advance_from_results()
 
     return results
 
@@ -127,7 +197,7 @@ def explain_cron():
     if not user:
         return jsonify({"message": "Nicht autorisiert"}), 401
     try:
-        data = request.get_json() or {}
+        data = request.get_json(silent=True) or {}
         expression = data.get("expression", "").strip()
 
         fields = expression.split()