feat: Add env vars to control log level and log format

### Describe the feature you'd like

Standard LOG_LEVEL setting would be handy, as would an env var to control log format. Specifically, it would be great to have an option to log to JSON format.

### Describe the benefits this would bring to existing Karakeep users

Easier handling of log messages and ingestion into log aggregation services.

### Can the goal of this request already be achieved via other means?

Nope

### Have you searched for an existing open/closed issue?

- [x] I have searched for existing issues and none cover my fundamental request

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by locating Karakeep's logging setup and environment-variable handling, then determine how log level and format are configured. Done should include documented environment variables for log level and JSON output, with tests covering both behaviors.

Written by the indexing model from the issue text.