perf: build debug log messages lazily so disabled debug logs cost nothing (#27834)

GLOBAL_LOG_LEVEL defaults to INFO, so every log.debug(...) in the backend is discarded, but the message is built first: 187 call sites interpolate their payload into an f-string before the logging call runs, so the work happens on every request and the result is thrown away. The worst one sits in process_chat_payload and stringifies the whole request body, full conversation history included, once per chat completion.

That one line with DEBUG disabled, CPython 3.12:

| conversation | payload | before   | after   |
| ------------ | ------- | -------- | ------- |
| 4 messages   | 1.2 kB  | 3.4 us   | 0.07 us |
| 20 messages  | 17 kB   | 24.8 us  | 0.07 us |
| 60 messages  | 123 kB  | 216.6 us | 0.07 us |

The lazy form log.debug('form_data: %s', form_data) hands the payload to record.getMessage(), which the InterceptHandler only reaches once a record has passed the level check. With DEBUG enabled the emitted lines are byte-identical, f'{x=}' sites included: those map to %r. MistralLoader._debug_log callers get the same treatment, since that wrapper already forwards *args.

backend/open_webui/utils/automations.py

@@ -667,7 +667,7 @@ async def _check_calendar_alerts(app) -> None:
                 CalendarEventUpdateForm(meta={'alerted_at': now_ns}),
             )
         except Exception:
-            log.debug(f'Failed to mark event {event.id} as alerted', exc_info=True)
+            log.debug('Failed to mark event %s as alerted', event.id, exc_info=True)
 
         # Send target notification if user has one configured
         try:
@@ -687,7 +687,7 @@ async def _check_calendar_alerts(app) -> None:
                 message=event.title,
             )
         except Exception:
-            log.debug(f'Failed to send notification for calendar alert {event.id}', exc_info=True)
+            log.debug('Failed to send notification for calendar alert %s', event.id, exc_info=True)
 
 
 async def _record_run(