feat(api): implement saved filters backend service and handlers

Add complete backend implementation for saved filters CRUD operations
with proper service layer architecture and RESTful API endpoints.

Service Layer (internal/services/filters.go):
- NewFiltersService() constructor following project patterns
- GetSavedFilters(): Retrieve all filters for user + resource type
- CreateSavedFilter(): Create filter with duplicate name validation
- UpdateSavedFilter(): Update filter with ownership verification
- DeleteSavedFilter(): Delete filter with user scoping

Business Logic:
- Filter name uniqueness enforced per user + resource type
- User ownership validation on all operations (JWT user_id)
- JSONB marshaling/unmarshaling for flexible filter storage
- Proper error wrapping with context messages

Handler Layer (internal/handlers/filters.go):
- NewFiltersHandler() constructor (receives db.Queries)
- GetSavedFilters: GET /api/saved-filters?resource_type=X
- CreateSavedFilter: POST /api/saved-filters
- UpdateSavedFilter: PUT /api/saved-filters/:id
- DeleteSavedFilter: DELETE /api/saved-filters/:id

Content Negotiation:
- Supports both JSON (API clients) and HTML (HTMX) responses
- wantsHTML() helper checks Accept header
- HX-Redirect header for HTMX form submissions
- Proper status codes (200, 201, 204, 400, 401, 404, 409)

Router Configuration:
- registerFiltersRoutes() function in internal/router/filters.go
- JWT middleware protection on all endpoints
- RESTful route structure: /api/saved-filters
- Registered in main router.go RegisterRoutes() function
- Added FiltersHandler to router.Config struct

Test Infrastructure:
- Added FiltersHandler to test server setup (test_helpers_test.go)
- FiltersHandler initialized in setupTestServer() function
- Router.Config includes FiltersHandler for integration tests

Code Quality:
- Follows PROJECT_GUIDELINES.md service layer patterns
- Uses database models (not custom domain models)
- JSONB returned as []byte (matches collections pattern)
- All errors wrapped with context using fmt.Errorf
- Handlers create services internally (not dependency injection)

Part of: Saved Filters Implementation (Phase 2: Backend)
Related: #saved-filters-feature

cmd/server/tests/test_helpers_test.go

@@ -466,6 +466,7 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 	services.WorkerInstance = worker
 	jobsHandler := handlers.NewJobsHandler(queries, worker)
 	collectionHandler := handlers.NewCollectionHandler(queries, libraryService, connManager)
+	filtersHandler := handlers.NewFiltersHandler(queries)
 	dashboardService := services.NewDashboardService(queries)
 	dashboardHandler := handlers.NewDashboardHandler(queries)
 	mediaHandler := handlers.NewMediaHandler(queries, libraryService, worker)
@@ -514,6 +515,7 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 		QueueHandler:          queueHandler,
 		SystemSettingsHandler: systemSettingsHandler,
 		CollectionHandler:     collectionHandler,
+		FiltersHandler:        filtersHandler,
 		DashboardHandler:      dashboardHandler,
 		DashboardService:      dashboardService,
 		OPDSHandler:           opdsHandler,