feat: add GET /api/saved-filters/:id endpoint with comprehensive tests

Implement missing GET endpoint for retrieving individual saved filters by ID.
This completes the CRUD API for saved filters and enables mobile/SPA clients
to fetch filter details on-demand.

Backend Implementation:
- Add GetSavedFilterByID() handler method (internal/handlers/filters.go)
  - Parse filter ID from URL parameter
  - Validate UUID format, return 400 for invalid IDs
  - Call service layer for business logic + ownership verification
  - Return 404 if filter not found or doesn't belong to user
  - Return 200 with filter object including filters JSONB

- Add GetSavedFilterByID() service method (internal/services/filters.go)
  - Call existing database query GetSavedFilterByID
  - Verify filter exists and belongs to user
  - Return descriptive error: "filter not found or access denied"
  - Reuses existing database query (no new SQL needed)

- Register GET /:id route (internal/router/filters.go)
  - Add route before existing GET "" route
  - Follows RESTful routing conventions

Integration Tests (cmd/server/tests/filters_test.go):
- Test success case: Create filter, retrieve by ID, verify data
- Test error case: Invalid UUID format returns 400
- Test error case: Non-existent filter returns 404
- Test error case: No authentication returns 401
- Test security case: Cross-user access returns 404 (not 403)
  - Admin creates filter, regular user tries to access
  - Uses setup.Token (admin) and setup.RegularToken
  - Verifies information leakage prevention

API Design:
- Endpoint: GET /api/saved-filters/:id
- Authentication: JWT token required
- Response format: SavedFilterResponse with filters as JSON
- Error responses: 400 (invalid ID), 401 (no auth), 404 (not found)
- Security: Returns 404 for cross-user access (hides existence)

Benefits:
- Completes CRUD API for saved filters
- Enables future mobile/SPA clients
- Follows existing handler/service/test patterns
- Comprehensive security testing
- No database changes required (reuses existing queries)

Follows PROJECT_GUIDELINES.md service layer architecture and testing patterns.

internal/services/filters.go

@@ -31,6 +31,19 @@ func (s *FiltersService) GetSavedFilters(ctx context.Context, userID uuid.UUID,
 	return filters, nil
 }
 
+// GetSavedFilterByID - Retrieve a single saved filter by ID
+func (s *FiltersService) GetSavedFilterByID(ctx context.Context, userID uuid.UUID, filterID uuid.UUID) (database.SavedFilters, error) {
+	filter, err := s.db.GetSavedFilterByID(ctx, database.GetSavedFilterByIDParams{
+		ID:     pgtype.UUID{Bytes: filterID, Valid: true},
+		UserID: pgtype.UUID{Bytes: userID, Valid: true},
+	})
+	if err != nil {
+		return database.SavedFilters{}, fmt.Errorf("filter not found or access denied: %w", err)
+	}
+
+	return filter, nil
+}
+
 // CreateSavedFilter - Create a new saved filter
 func (s *FiltersService) CreateSavedFilter(ctx context.Context, userID uuid.UUID, name string, resourceType string, filters map[string]string) (database.SavedFilters, error) {
 	// Business logic: Validate filter name uniqueness per user + resource type