Fix local_datetime filter to handle string datetime values

The filter now accepts both datetime objects and string representations.
This fixes AttributeError when datetime fields are stored/returned as
strings instead of datetime objects from the database.

Changes:
- Added string parsing logic for common datetime formats
- Tries multiple format patterns (ISO, standard, with/without microseconds)
- Returns original string if parsing fails (graceful degradation)
- Maintains full datetime object support

Fixes errors on:
- /customers (autotask_last_sync_at)
- /logging (created_at)
- /admin/all-mail (received_at, parsed_at)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

containers/backupchecks/src/backend/app/__init__.py

@@ -68,7 +68,7 @@ def create_app():
         """Convert UTC datetime to UI timezone and format as string.
 
         Args:
-            utc_dt: datetime object in UTC (or None)
+            utc_dt: datetime object in UTC, string representation, or None
             format: strftime format string (default: '%Y-%m-%d %H:%M:%S')
 
         Returns:
@@ -79,6 +79,30 @@ def create_app():
 
         from datetime import datetime
 
+        # If input is already a string, try to parse it first
+        if isinstance(utc_dt, str):
+            utc_dt = utc_dt.strip()
+            if not utc_dt:
+                return ""
+            try:
+                # Try common datetime formats
+                for fmt in [
+                    "%Y-%m-%d %H:%M:%S.%f",  # With microseconds
+                    "%Y-%m-%d %H:%M:%S",     # Without microseconds
+                    "%Y-%m-%dT%H:%M:%S.%fZ", # ISO format with Z
+                    "%Y-%m-%dT%H:%M:%S",     # ISO format without Z
+                ]:
+                    try:
+                        utc_dt = datetime.strptime(utc_dt, fmt)
+                        break
+                    except ValueError:
+                        continue
+                else:
+                    # If parsing failed, return original string
+                    return str(utc_dt)
+            except Exception:
+                return str(utc_dt)
+
         try:
             from zoneinfo import ZoneInfo
         except ImportError: