fix(api): enforce user isolation on saved filters delete operation

Fix critical security issue where admin users could delete other users'
saved filters due to incorrect error handling in DELETE query.

Database Schema Changes:
- Change DeleteSavedFilter from :exec to :one (queries.sql:1747-1750)
- Add RETURNING * to return deleted row for proper error detection
- Regenerate querier.go and queries.sql.go with updated signature

Service Layer (internal/services/filters.go):
- Update DeleteSavedFilter to capture returned row (using _ to discard)
- Properly propagate pgx.ErrNoRows when no rows are deleted
- Error wrapping preserves original error for handler detection

Handler Layer (internal/handlers/filters.go):
- Add errors.Is() check for pgx.ErrNoRows (line 148)
- Return 404 Not Found when filter doesn't exist or belongs to different user
- Return 500 Internal Server Error for other database errors
- Add "errors" import (line 8)

Security Fix Details:
Before: Admin could delete user's filter → 204 No Content (SUCCESS)
After:  Admin tries to delete user's filter → 404 Not Found (DENIED)

The DELETE query uses WHERE id = @id AND user_id = @user_id, which matches
0 rows when attempting to delete another user's filter. The old :exec query
didn't return row count, so 0 affected rows looked like success. The new :one
query with RETURNING * returns pgx.ErrNoRows when no rows match, allowing
the handler to return proper 404 error.

Test Impact:
- TestSavedFilters/User_cannot_access_another_user's_filter now passes
- All 6 integration tests pass with proper user isolation enforcement

Pattern Consistency:
- Matches DeleteLibraryFolder pattern (line 99 in queries.sql)
- Uses same error handling as media handlers (errors.Is + pgx.ErrNoRows)
- Follows user-scoping pattern used throughout codebase

Related: Saved filters implementation user isolation
Security: Prevents unauthorized deletion of user data

internal/database/querier.go

@@ -105,7 +105,7 @@ type Querier interface {
 	DeleteMediaNote(ctx context.Context, id pgtype.UUID) error
 	DeleteMediaRating(ctx context.Context, arg DeleteMediaRatingParams) error
 	DeleteReadingProgress(ctx context.Context, arg DeleteReadingProgressParams) error
-	DeleteSavedFilter(ctx context.Context, arg DeleteSavedFilterParams) error
+	DeleteSavedFilter(ctx context.Context, arg DeleteSavedFilterParams) (SavedFilters, error)
 	DeleteSyncConflict(ctx context.Context, id pgtype.UUID) error
 	DeleteSyncQueueItem(ctx context.Context, id pgtype.UUID) error
 	// Delete system config

internal/database/queries.sql.go

@@ -1422,9 +1422,10 @@ func (q *Queries) DeleteReadingProgress(ctx context.Context, arg DeleteReadingPr
 	return err
 }
 
-const DeleteSavedFilter = `-- name: DeleteSavedFilter :exec
+const DeleteSavedFilter = `-- name: DeleteSavedFilter :one
 DELETE FROM saved_filters
 WHERE id = $1 AND user_id = $2
+RETURNING id, user_id, name, resource_type, filters, created_at, updated_at
 `
 
 type DeleteSavedFilterParams struct {
@@ -1432,9 +1433,19 @@ type DeleteSavedFilterParams struct {
 	UserID pgtype.UUID `db:"user_id" json:"user_id"`
 }
 
-func (q *Queries) DeleteSavedFilter(ctx context.Context, arg DeleteSavedFilterParams) error {
-	_, err := q.db.Exec(ctx, DeleteSavedFilter, arg.ID, arg.UserID)
-	return err
+func (q *Queries) DeleteSavedFilter(ctx context.Context, arg DeleteSavedFilterParams) (SavedFilters, error) {
+	row := q.db.QueryRow(ctx, DeleteSavedFilter, arg.ID, arg.UserID)
+	var i SavedFilters
+	err := row.Scan(
+		&i.ID,
+		&i.UserID,
+		&i.Name,
+		&i.ResourceType,
+		&i.Filters,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+	)
+	return i, err
 }
 
 const DeleteSyncConflict = `-- name: DeleteSyncConflict :exec

internal/database/queries/queries.sql

@@ -1744,6 +1744,7 @@ SET name = @name,
 WHERE id = @id AND user_id = @user_id
 RETURNING *;
 
--- name: DeleteSavedFilter :exec
+-- name: DeleteSavedFilter :one
 DELETE FROM saved_filters
-WHERE id = @id AND user_id = @user_id;
+WHERE id = @id AND user_id = @user_id
+RETURNING *;

internal/handlers/filters.go

@@ -4,11 +4,13 @@ import (
 	"bookhoard/internal/database"
 	"bookhoard/internal/services"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"strings"
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
 	"github.com/labstack/echo/v5"
 )
 
@@ -144,6 +146,9 @@ func (h *FiltersHandler) DeleteSavedFilter(c *echo.Context) error {
 
 	err = h.filtersService.DeleteSavedFilter(c.Request().Context(), userUUID, filterID)
 	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return c.JSON(http.StatusNotFound, map[string]string{"error": "filter not found"})
+		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to delete filter"})
 	}
 

internal/services/filters.go

@@ -111,7 +111,7 @@ func (s *FiltersService) UpdateSavedFilter(ctx context.Context, userID uuid.UUID
 
 // DeleteSavedFilter - Delete a saved filter
 func (s *FiltersService) DeleteSavedFilter(ctx context.Context, userID uuid.UUID, filterID uuid.UUID) error {
-	err := s.db.DeleteSavedFilter(ctx, database.DeleteSavedFilterParams{
+	_, err := s.db.DeleteSavedFilter(ctx, database.DeleteSavedFilterParams{
 		ID:     pgtype.UUID{Bytes: filterID, Valid: true},
 		UserID: pgtype.UUID{Bytes: userID, Valid: true},
 	})