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/routers/audio.py

@@ -486,7 +486,7 @@ async def _tts_transformers(request, payload, file_path, file_body_path, user):
     try:
         idx = embeddings['filename'].index(model_name)
     except (ValueError, KeyError):
-        log.debug(f'Speaker embedding not found for {model_name}, using default index {idx}')
+        log.debug('Speaker embedding not found for %s, using default index %s', model_name, idx)
 
     def _run_pipeline():
         speaker_embedding = torch.tensor(embeddings[idx]['xvector']).unsqueeze(0)
@@ -1303,7 +1303,7 @@ async def get_available_models(request: Request) -> list[dict]:
                     data = await resp.json()
                     available_models = data.get('models', [])
             except Exception as e:
-                log.debug(f'/audio/models not available, trying /models fallback: {e}')
+                log.debug('/audio/models not available, trying /models fallback: %s', e)
                 try:
                     async with session.get(
                         f'{base_url}/models',