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

docs/api/collections/add_auto_assign_rule.md

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+# Add Auto-Assign Rule
+
+Add an automatic book assignment rule to a collection.
+
+**Endpoint**: `POST /api/collections/{id}/rules`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| id | string (UUID) | Yes | Collection UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| field | string | Yes | Field to match on (genre, author, series, language, publisher, copyright_year, tags) |
+| operator | string | Yes | Comparison operator (equals, not_equals, contains, not_contains, starts_with, ends_with, greater_than, less_than) |
+| value | string/number | Yes | Value to compare against |
+| priority | integer | No | Rule priority (1 = highest, default: 1) |
+| enabled | boolean | No | Whether rule is active (default: true) |
+
+### Supported Fields
+
+| Field | Type | Description |
+|-------|------|-------------|
+| genre | string | Book genre |
+| author | string | Book author |
+| series | string | Book series name |
+| language | string | Book language |
+| publisher | string | Publisher name |
+| copyright_year | number | Publication year (numeric comparison) |
+| tags | string | Book tags |
+
+### Supported Operators
+
+| Operator | Type | Description |
+|----------|------|-------------|
+| equals | all | Exact match |
+| not_equals | all | Not equal |
+| contains | string | Contains substring (case-insensitive) |
+| not_contains | string | Does not contain |
+| starts_with | string | Starts with (case-insensitive) |
+| ends_with | string | Ends with (case-insensitive) |
+| greater_than | number | Greater than |
+| less_than | number | Less than |
+
+### Example Request
+
+```json
+{
+  "field": "genre",
+  "operator": "equals",
+  "value": "Science Fiction",
+  "priority": 1,
+  "enabled": true
+}
+```
+
+## Response (201 Created)
+
+```json
+{
+  "id": "rule-uuid-here",
+  "field": "genre",
+  "operator": "equals",
+  "value": "Science Fiction",
+  "priority": 1,
+  "enabled": true,
+  "created_at": "2026-02-01T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request (validation failed) |
+| 401 | Authentication required |
+| 404 | Collection not found |
+| 500 | Internal server error |
+
+## Try It Out