docs: add GET /:id endpoint documentation and update implementation plan

Add comprehensive documentation for GET /api/saved-filters/:id endpoint
including Bruno API collection, developer API docs, user documentation,
and implementation plan with frontend integration phase.

Bruno API Collection (bruno/saved-filters/Get Saved Filter By ID.yml):
- New Bruno request file for GET /:id endpoint
- Includes comprehensive documentation with examples
- Documents all status codes (200, 400, 401, 404)
- Provides example curl commands and use cases
- Uses variable placeholders ({{base_url}}, {{filter_id}})
- Follows existing Bruno YAML patterns

API Documentation (docs/developer/api/saved-filters/index.md):
- Added GET /api/saved-filters/:id endpoint documentation
- Example request with UUID parameter
- Example response showing filter object structure
- Error responses documented (400, 401, 404)
- Use cases: Mobile apps, SPAs, editing, verification

User Documentation (docs/user/library-browsing.md):
- Updated "Loading Saved Filters" section
- Removed "feature coming soon" language
- Added step-by-step instructions for loading filters
- Added tips section with visual indicators
- Added "Managing Saved Filters" section
- Added "Common Use Cases" (genre, author, series)
- Emphasizes instant feedback (no page reload)

Implementation Plan (GET_SAVED_FILTER_BY_ID_IMPLEMENTATION.md):
- Added Phase 7: User Documentation Update
- Added Phase 8: Frontend Integration (bookshelf.ts)
  - Shows loadFilter() implementation
  - Hybrid Alpine.js + HTMX approach
  - Maintains SSR-first principles
  - API call on user interaction, not page load
  - Populates hidden form fields
  - Triggers HTMX to apply filter
- Updated Summary of Changes: 7 files, ~344 lines
- Updated Checklist with frontend and user docs tasks
- Added frontend testing tasks

SSR-First Compliance:
- Initial page load: Server renders everything (no API calls)
- User interaction only: API called when user clicks filter
- No async x-init data fetching
- Progressive enhancement maintained

Documentation Structure:
- Developer docs: API reference for integration
- User docs: Step-by-step usage instructions
- Bruno: API contract testing
- Implementation plan: Complete development guide

All documentation follows established patterns and includes examples.

docs/developer/api/saved-filters/index.md

@@ -104,3 +104,46 @@ Delete a saved filter.
 
 **Error Responses:**
 - 404 Not Found - Filter doesn't exist or doesn't belong to user
+
+### GET /api/saved-filters/:id
+
+Retrieve a single saved filter by ID.
+
+**URL Parameters:**
+- `id` (UUID, required) - Filter ID to retrieve
+
+**Response:** Single saved filter object (HTTP 200)
+
+**Example Request:**
+```bash
+curl -H "Authorization: Bearer YOUR_TOKEN" \\
+  "http://localhost:8765/api/saved-filters/550e8400-e29b-41d4-a716-446655440000"
+```
+
+**Example Response:**
+```json
+{
+  "id": "550e8400-e29b-41d4-a716-446655440000",
+  "name": "My Sci-Fi Books",
+  "resource_type": "media-items",
+  "filters": {
+    "genre_filter": "Science Fiction",
+    "sort": "title ASC"
+  },
+  "created_at": "2024-03-20T12:00:00Z",
+  "updated_at": "2024-03-20T12:00:00Z"
+}
+```
+
+**Error Responses:**
+- 400 Bad Request - Invalid filter ID format
+- 401 Unauthorized - Invalid or missing authentication token
+- 404 Not Found - Filter doesn't exist or doesn't belong to user
+
+**Use Cases:**
+- Mobile apps: Fetch filter details on-demand
+- SPAs: Load filter data without page reload
+- Editing: Pre-fill filter update form
+- Verification: Check filter exists before operations
+
+---