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

[kunyoungp]

Summary

• Added Tag model (id, name String(50) unique, timestamps) and note_tags association table
• Configured many-to-many relationship: Note.tags ↔ Tag.notes with lazy="selectin" and CASCADE deletes
• Added TagCreate, TagRead schemas; updated NoteRead to include tags: list[TagRead] = []
• New tags router: GET /tags/, POST /tags/ (201, 409 on duplicate), DELETE /tags/{tag_id} (204, 404)
• Added tag endpoints on notes: POST /notes/{note_id}/tags/{tag_id} (idempotent attach), DELETE /notes/{note_id}/tags/{tag_id} (detach)
• Registered tags router in main.py
• Updated seed.sql with tags and note_tags CREATE TABLE statements

Test plan

• test_create_and_list_tags — create and list tags
• test_create_duplicate_tag_returns_409 — duplicate name returns 409
• test_delete_tag — delete returns 204
• test_delete_tag_not_found — delete non-existent returns 404
• test_attach_tag_to_note — attach tag, verify in response
• test_attach_tag_idempotent — attaching twice results in single tag
• test_detach_tag_from_note — detach tag, verify removed
• test_note_read_includes_tags — GET note includes tags field
• test_cascade_delete_tag_cleans_note_tags — deleting tag removes from note associations
• All 12 tests pass (make test)

Notable tradeoffs

• lazy="selectin" eagerly loads tags with notes (avoids N+1 but adds overhead for tag-less queries)
• Tag attach is idempotent (no error if already attached); detach silently succeeds if not attached
• 409 Conflict for duplicate tag names uses SQLAlchemy IntegrityError catch rather than pre-check query

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