docs: add API pagination, filtering and sorting specification

docs/api-pagination-filtering-sorting.md

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+# API Pagination, Filtering, and Sorting
+
+This document defines how GreenCode backend APIs should implement pagination, filtering, and sorting across all list endpoints. Frontend developers and contributors should follow these conventions when building or consuming list-based APIs.
+
+---
+
+##  Pagination
+
+All list endpoints must support cursor- or offset-based pagination using the following query parameters:
+
+| Parameter | Type    | Default | Description                        |
+|-----------|---------|---------|------------------------------------|
+| `page`    | integer | `0`     | Zero-indexed page number           |
+| `size`    | integer | `20`    | Number of results per page (max 100) |
+
+### Example Request
+### Example Response
+
+```json
+{
+  "content": [...],
+  "page": 0,
+  "size": 10,
+  "totalElements": 87,
+  "totalPages": 9,
+  "last": false
+}
+```
+
+---
+
+##  Filtering
+
+Filters are passed as query parameters. Multiple filters are combined with AND logic.
+
+| Parameter     | Example Value     | Description                        |
+|---------------|-------------------|------------------------------------|
+| `status`      | `active`          | Filter by project or resource status |
+| `category`    | `sustainability`  | Filter by category tag             |
+| `createdAfter`| `2024-01-01`      | Filter records created after date  |
+| `createdBefore`| `2024-12-31`     | Filter records created before date |
+| `search`      | `green farm`      | Full-text search on name/description |
+
+### Example Request
+---
+
+##  Sorting
+
+Sorting is controlled by `sort` and `direction` query parameters.
+
+| Parameter   | Values                        | Default      |
+|-------------|-------------------------------|--------------|
+| `sort`      | `createdAt`, `name`, `status` | `createdAt`  |
+| `direction` | `asc`, `desc`                 | `desc`       |
+
+### Example Request
+---
+
+##  Combined Example
+
+Retrieve the second page of active sustainability projects, sorted by name ascending:
+---
+
+##  Backend Implementation Notes (Spring Boot)
+
+Use Spring Data's `Pageable` interface for pagination and sorting:
+
+```java
+@GetMapping("/projects")
+public Page<ProjectDTO> getProjects(
+    @RequestParam(defaultValue = "0") int page,
+    @RequestParam(defaultValue = "20") int size,
+    @RequestParam(defaultValue = "createdAt") String sort,
+    @RequestParam(defaultValue = "desc") String direction,
+    @RequestParam(required = false) String status,
+    @RequestParam(required = false) String search
+) {
+    Pageable pageable = PageRequest.of(page, size,
+        Sort.by(Sort.Direction.fromString(direction), sort));
+    return projectService.findAll(status, search, pageable);
+}
+```
+
+---
+
+##  Frontend Implementation Notes (React)
+
+Use Axios with query parameters:
+
+```javascript
+const fetchProjects = async ({ page = 0, size = 10, sort = 'createdAt', direction = 'desc', status, search }) => {
+  const params = { page, size, sort, direction };
+  if (status) params.status = status;
+  if (search) params.search = search;
+
+  const response = await axios.get('/api/projects', { params });
+  return response.data;
+};
+```
+
+---
+
+##  Checklist for Contributors
+
+When adding a new list endpoint:
+- [ ] Support `page` and `size` query params
+- [ ] Return pagination metadata in response body
+- [ ] Support at least `search` filter
+- [ ] Support `sort` and `direction` params
+- [ ] Document the endpoint in Swagger/OpenAPI annotations