[Week7 - jbk] Task3: Add Tag model with many-to-many relationship

[Candyroom136]

Summary

• Added Tag model with unique name constraint
• Created note_tags association table for many-to-many relationship between Notes and Tags
• Added 5 new endpoints: list/create/delete tags, attach/detach tags to notes
• Updated NoteRead schema to include tags

Testing

$ make test
10 passed (3 existing + 7 new)

• Tests cover: tag CRUD, attach/detach, 404 handling, empty tags default, validation

Tradeoffs & Follow-ups

• Tag names are unique — duplicate creation will fail with 500 (could add friendlier error)
• No pagination on tags endpoint (assuming small number of tags)
• Deleting a tag doesn't cascade-notify notes that had it

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