* 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.
