feat: add LOG_FORMAT=json for structured JSON logging (#21747)

* feat: add LOG_FORMAT env var with JSON formatter for early logging

Introduce LOG_FORMAT environment variable (set to "json" to enable).
When active, logging.basicConfig() uses a JSONFormatter that outputs
single-line JSON objects with fields: ts, level, msg, caller, error,
stacktrace. This covers all log messages emitted during module imports
before Loguru's start_logger() takes over.

* feat: add JSON sink for Loguru when LOG_FORMAT=json

Add _json_sink() as a Loguru sink function that writes single-line JSON
to stdout. In start_logger(), conditionally use the JSON sink instead of
the plain-text stdout_format when LOG_FORMAT is set to "json".

* feat: suppress ASCII banner and fix alembic logging in JSON mode

- Wrap the ASCII art banner print in main.py with a LOG_FORMAT != "json"
  guard so JSON output stays machine-parseable.
- Skip alembic's fileConfig() call in migrations/env.py when
  LOG_FORMAT=json to prevent it from replacing the JSON log handlers
  installed during early startup.

backend/open_webui/main.py

@@ -503,6 +503,7 @@ from open_webui.env import (
     WEBUI_ADMIN_PASSWORD,
     WEBUI_ADMIN_NAME,
     ENABLE_EASTER_EGGS,
+    LOG_FORMAT,
 )
 
 
@@ -581,7 +582,8 @@ class SPAStaticFiles(StaticFiles):
                 raise ex
 
 
-print(rf"""
+if LOG_FORMAT != "json":
+    print(rf"""
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