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/files.py

@@ -103,7 +103,7 @@ def _cleanup_local_cache(file_path: str) -> None:
         local_path = os.path.join(UPLOAD_DIR, local_filename)
         if os.path.isfile(local_path):
             os.remove(local_path)
-            log.debug(f'Cleaned up local cache: {local_path}')
+            log.debug('Cleaned up local cache: %s', local_path)
     except OSError as e:
         log.warning(f'Failed to clean up local cache for {file_path}: {e}')
 
@@ -1021,7 +1021,7 @@ async def delete_file_by_id(
                 if file.hash:
                     await ASYNC_VECTOR_DB_CLIENT.delete(collection_name=knowledge.id, filter={'hash': file.hash})
             except Exception as e:
-                log.debug(f'KB embedding cleanup for {knowledge.id}: {e}')
+                log.debug('KB embedding cleanup for %s: %s', knowledge.id, e)
 
         result = await Files.delete_file_by_id(id, db=db)
         if result: