fix(fonts): address PR #190 code review findings

[sharkAndshark]

Summary

Addresses code review findings from PR #190.

Fixes

1. Timestamp consistency — Use CURRENT_TIMESTAMP for created_at and published_at in SQL instead of Rust-side Utc::now().to_rfc3339(), matching the rest of the codebase and eliminating DuckDB implicit RFC3339 to TIMESTAMP parsing fragility.

2. Deduplicated test-mode workspace bootstrap — Extract workspace::ensure_test_mode_workspace() shared function, eliminating ~100 lines of identical bootstrap code duplicated between font_handlers.rs and upload.rs.

3. Deduplicated FontItem row mapping — Extract read_font_row() helper for the 13-column row mapping previously duplicated in list_fonts and get_font.

4. Error logging — update_font_error now logs DB errors via warn! instead of silently discarding them with let _ =.

5. Font file extension — Save uploaded fonts with their original extension (e.g., original.ttf instead of original) for easier filesystem inspection.

Note on race condition (Item 1 from review)

The review flagged a TOCTOU race in publish_font between the SELECT EXISTS check and the UPDATE. This race is already prevented by the application-level Mutex<Connection> - the lock is held continuously from the conflict check through the update. No additional fix needed.

Test results

• 84 unit tests pass
• 150 API integration tests pass
• 7 font API tests pass
• 6 PostGIS tests pass
• 33 frontend unit tests pass
• 50 E2E tests pass