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.github/instructions/api.md

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+# API Instructions
+
+## Scope
+- Applies to Fiber handlers and route registration under `internal/handlers` and `internal/server/routes.go`.
+
+## Rules
+- Keep API contracts stable; if changing request/response shape, document backward compatibility in PR notes.
+- Prefer snake_case JSON request fields for backend payloads and accept legacy aliases only when needed.
+- Validate required fields explicitly in handlers and return clear `400` errors with actionable messages.
+- Return `404` for missing resources, `500` only for unexpected server/database failures.
+- Keep route naming consistent (`/resource`, `/resource/:id`) and group related routes in `routes.go`.
+
+## Implementation Notes
+- Use `database.DB()` directly in handlers unless a service layer exists for that feature.
+- Preload only required relations for performance-sensitive list endpoints.
+- Keep list endpoints sorted deterministically (e.g., `position asc, id asc`).