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/env.py

@@ -5,6 +5,9 @@ import os
 import pkgutil
 import sys
 import shutil
+import traceback
+from datetime import datetime, timezone
+from typing import Any
 from uuid import uuid4
 from pathlib import Path
 from cryptography.hazmat.primitives import serialization
@@ -72,9 +75,51 @@ except Exception:
 # LOGGING
 ####################################
 
+_LEVEL_MAP = {
+    "DEBUG": "debug",
+    "INFO": "info",
+    "WARNING": "warn",
+    "ERROR": "error",
+    "CRITICAL": "fatal",
+}
+
+
+class JSONFormatter(logging.Formatter):
+    """Format log records as single-line JSON objects for structured logging."""
+
+    def format(self, record: logging.LogRecord) -> str:
+        log_entry: dict[str, Any] = {
+            "ts": datetime.fromtimestamp(record.created, tz=timezone.utc).isoformat(
+                timespec="milliseconds"
+            ),
+            "level": _LEVEL_MAP.get(record.levelname, record.levelname.lower()),
+            "msg": record.getMessage(),
+            "caller": record.name,
+        }
+
+        if record.exc_info and record.exc_info[0] is not None:
+            log_entry["error"] = "".join(
+                traceback.format_exception(*record.exc_info)
+            ).rstrip()
+        elif record.exc_text:
+            log_entry["error"] = record.exc_text
+
+        if record.stack_info:
+            log_entry["stacktrace"] = record.stack_info
+
+        return json.dumps(log_entry, ensure_ascii=False, default=str)
+
+
+LOG_FORMAT = os.environ.get("LOG_FORMAT", "").lower()
+
 GLOBAL_LOG_LEVEL = os.environ.get("GLOBAL_LOG_LEVEL", "").upper()
 if GLOBAL_LOG_LEVEL in logging.getLevelNamesMapping():
-    logging.basicConfig(stream=sys.stdout, level=GLOBAL_LOG_LEVEL, force=True)
+    if LOG_FORMAT == "json":
+        _handler = logging.StreamHandler(sys.stdout)
+        _handler.setFormatter(JSONFormatter())
+        logging.basicConfig(handlers=[_handler], level=GLOBAL_LOG_LEVEL, force=True)
+    else:
+        logging.basicConfig(stream=sys.stdout, level=GLOBAL_LOG_LEVEL, force=True)
 else:
     GLOBAL_LOG_LEVEL = "INFO"