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

[HyungjoonYang]

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 note-tag endpoints: POST /notes/{note_id}/tags/{tag_id} (idempotent attach), DELETE /notes/{note_id}/tags/{tag_id} (detach)
• Registered tags router in main.py

Test plan

• Create and list tags
• Duplicate tag name → 409
• Delete tag → 204, nonexistent → 404
• Attach tag to note, verify in response
• Attach tag twice → idempotent (single tag)
• Detach tag from note
• 404 for nonexistent note/tag on attach
• GET note includes empty tags by default
• Cascade: deleting tag removes from note associations
• All 15 tests pass

Notable tradeoffs

• lazy="selectin" eagerly loads tags with notes (avoids N+1 but adds overhead for tag-less queries)
• Tag attach is idempotent; detach silently succeeds if not attached
• 409 Conflict for duplicate tags uses SQLAlchemy IntegrityError catch
• No tag rename endpoint — delete + recreate works for now

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