feat: mock endpoint — configure response per endpoint
- Add mock_status, mock_headers, mock_body columns to endpoints schema
- Add MockConfig struct and UpsertMock to Repository interface
- Implement UpsertMock in sqliteRepo (INSERT ON CONFLICT UPDATE)
- Update GetEndpoint to return mock configuration when present
- Add PUT /e/{endpoint_id}/mock handler with JSON body {status, headers, body}
- Add catch-all /{endpoint_id} handler returning mock or default 200
- Header sanitization: only allow Content-Type, Cache-Control, X-*
- Wire handler into cmd/hatch main with SQLite store
- Fix Dockerfile: COPY go.sum for reproducible builds
- Tests: store mock CRUD, handler mock config + header sanitization
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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