[TECHDEBT] Document Python 3.9+ requirement and DB connection patterns

[deucebucket]

Tech Debt

From vibe-check review on PR #134:

1. Python version requirement: Path.is_relative_to() (used in pipeline layers) requires Python 3.9+. Dockerfile uses 3.11 so no issue, but should be documented in requirements.txt or pyproject.toml for anyone running outside Docker.

2. DB connection pattern documentation: get_db() returns a new connection each time (standard Flask-SQLite). Multiple get_db() calls in the same function scope are safe but could confuse future contributors. Consider adding a brief comment in worker.py explaining the pattern.

3. api_stats() query complexity: The queue count query now has a JOIN + filters. A comment explaining why (matching process_queue filters) would help future maintainers.

Low priority - none of these cause runtime issues.

[deucebucket]

Additional items from PR #144 vibe-check

4. find_existing_author_folder() OSError logging level (path_safety.py): Currently logs at debug when directory listing fails (permissions, disk error). Should be warning so filesystem access failures are visible when investigating dedup issues.

5. find_existing_author_folder() docstring (path_safety.py): Add parameter type documentation for lib_path and target_author to match other documented functions in the module.

[deucebucket]

Progress Update

Completed:

• ✅ Item 3: api_stats() query comment added (line 8562 explains JOIN + filters matching process_queue)

Remaining:

• ⬜ Item 1: Document Python 3.9+ requirement (no python_requires in requirements.txt, no pyproject.toml)
• ⬜ Item 2: Add DB connection pattern comment in worker.py explaining get_db() returns new connection each call

[deucebucket]

Shipped in PR #145. Python 3.9+ requirement documented, get_db() pattern explained, param docs and logging level updated per vibe-check.