Fix: normalize tags to lowercase across all write paths and existing …

[hunter-read]

…data

Summary

• Add startup DB migration that lowercases and deduplicates tags in all seven tag-bearing tables, resolving collisions like "Draw Steel" vs "draw steel" in existing databases
• Add Pydantic field validators to MapUpdate, FolderTagsUpdate (maps & tokens), BookFolderUpdate, and GameSystemUpdate to enforce lowercase and deduplication on every API write
• Normalize tags read from tags.json in the indexer before storing
• Fix MapsView tag filter to build sets from lowercased values so clicking a filter pill matches maps regardless of stored casing

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Docs / configuration only

Testing

• Backend tests pass (pytest -q)
• Frontend tests pass (npm test)
• Tested manually in the browser

Notes for reviewer