Feature: User-defined tags for annotations

[texasbe2trill]

Summary

Users should be able to tag or categorize their annotations (e.g., "favorite", "to revisit", "quote", "concept") for easier filtering and retrieval. Currently annotations are only organized by book and chapter.

Requirements

• Add an optional tags: list[str] field to the Annotation model
• Add a tagging UI in the Streamlit annotations view (multi-select or free-text input)
• Persist tags locally (JSON sidecar file or SQLite — not modifying the Kobo database)
• Add tag-based filtering to the annotations view
• Add tag column to CSV export
• Add tests for tag persistence and filtering

Considerations

• Tags should persist across sessions without modifying the original Kobo database
• A simple JSON file (keyed by annotation hash) would be the lightest approach
• The AI insights pipeline could eventually suggest tags automatically