[Week7 - hjy] Task1: Add endpoints and input validation

[HyungjoonYang]

Summary

• Added DELETE /notes/{note_id} endpoint (204 No Content, 404 if missing)
• Added GET /action-items/{item_id} and DELETE /action-items/{item_id} endpoints
• Added Pydantic Field constraints for input validation:
  • NoteCreate: title (1-200 chars, non-blank), content (non-blank)
  • ActionItemCreate: description (non-blank)
  • NotePatch / ActionItemPatch: same constraints on optional fields
• Added query param validation: skip >= 0, 1 <= limit <= 200
• Invalid sort field now returns 400 instead of silent fallback

Test plan

• DELETE note → 204, subsequent GET → 404
• DELETE nonexistent note → 404
• GET /action-items/{id} → 200, nonexistent → 404
• DELETE action item → 204, subsequent GET → 404
• Empty/whitespace title/content/description → 422
• Title > 200 chars → 422
• Negative skip, zero/excess limit → 422
• Invalid sort field → 400
• All 27 tests pass

Notable tradeoffs

• DELETE returns 204 with no body (REST convention)
• Whitespace-only validation uses field_validator on top of min_length for stricter checking
• Hard delete, no soft-delete/undo

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