docs: add collections API endpoint files (Phase 2 completion)

docs/api/collections/bulk_assign.md

@@ -0,0 +1,52 @@
+# Bulk Assign Books
+
+Add multiple books to a collection at once.
+
+**Endpoint**: `POST /api/collections/{id}/books`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| id | string (UUID) | Yes | Collection UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| book_ids | array of UUID | Yes | Array of book IDs to add |
+
+### Example Request
+
+```json
+{
+  "book_ids": [
+    "660e8400-e29b-41d4-a716-446655440000",
+    "770e8400-e29b-41d4-a716-446655440000",
+    "880e8400-e29b-41d4-a716-446655440000"
+  ]
+}
+```
+
+## Response (204 No Content)
+
+Books added to collection successfully. No response body.
+
+## Notes
+
+- Books already in the collection are ignored
+- This is more efficient than adding books individually
+- Maximum 1000 books per request
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request (validation failed) |
+| 401 | Authentication required |
+| 404 | Collection or book(s) not found |
+| 500 | Internal server error |
+
+## Try It Out