refactor: update database schema and enhance image processing

Database Changes:
- Rename implementations table to impls
- Add preview_thumb, preview_html fields for thumbnails and interactive charts
- Add python_version, library_version to track generation context
- Remove GCS fallback in API (database URLs only)
- Filter specs endpoint to only return specs with implementations

Image Processing:
- Add pngquant optimization for PNG compression (~67% size reduction)
- Add pyplots.ai branded watermark with JetBrains Mono font
- Add HTML watermark support for interactive charts (Altair, etc.)
- Brand colors: Python blue (#3776AB), yellow (#FFD43B), dark (#1f2937)
- Gray shadow (2px offset) for watermark readability

Cleanup:
- Remove disabled workflow files (.yml.disabled)
- Update README with correct file structure
- Update database.md with current schema
- Update repository.md with GCS file naming
- Fix ruff linting issues

Author: MarkusNeusinger