Improve autotagger efficiency and configuration

[mitcdh]

Summary

• refactor the single script into a tested, maintainable package while keeping the existing entry point
• add configurable OpenAI-compatible endpoints, provider compatibility controls, custom prompts, and optional Azure support
• add bounded concurrent analysis, retry handling, resumable checkpoints, cost and photo limits, safe dry-runs, and tag preservation
• improve Flickr pagination, selection controls, validation, atomic reports, and partial-update handling
• lock dependencies and update CI, the manual autotag workflow, devcontainer setup, and documentation

Compatibility

The existing GitHub Actions setup continues to work with only:

• FLICKR_API_KEY
• FLICKR_API_SECRET
• FLICKR_OAUTH_TOKEN
• OPENAI_API_KEY

The standard workflow defaults to OpenAI and requires no new variables. Existing JSON contents in FLICKR_OAUTH_TOKEN use the same six fields and remain compatible.

Validation

• Ruff lint and formatting checks
• 35 tests on Python 3.11
• 35 tests on Python 3.12
• Actionlint validation for both GitHub workflow files