feat(router): Add admin processing issues UI route

Add frontend route /admin/libraries/:id/issues to display processing issues
management page for a specific library.

internal/router/frontend.go:
- Register GET /admin/libraries/:id/issues with admin middleware
- Fetch processing issue stats from database
- List processing issues for the library
- Convert database models to template types
- Render AdminProcessingIssues template with issues and stats

This provides the admin UI for viewing and managing processing errors that
occur during media scanning and import workflows.

internal/router/frontend.go

@@ -818,6 +818,62 @@ func registerFrontendRoutes(cfg *Config) {
 		return c.HTML(http.StatusOK, buf.String())
 	}))
 
+	frontendProtected.GET("/admin/libraries/:id/issues", handlers.AdminMiddleware(func(c *echo.Context) error {
+		user, err := getTemplateUserWithTheme(c, cfg)
+		if err != nil {
+			return renderErrorPage(c, "Error loading user", "user_load_error")
+		}
+		libraryID := c.Param("id")
+		libUUID, err := uuid.Parse(libraryID)
+		if err != nil {
+			return renderErrorPage(c, "Invalid library ID", "invalid_id")
+		}
+		// Fetch stats
+		stats, err := cfg.Queries.GetProcessingIssueStats(
+			c.Request().Context(),
+			uuidToPGType(libUUID),
+		)
+		if err != nil {
+			return renderErrorPage(c, "Error loading stats", "stats_load_error")
+		}
+		// Fetch issues
+		dbIssues, err := cfg.Queries.ListProcessingIssuesByLibrary(
+			c.Request().Context(),
+			uuidToPGType(libUUID),
+		)
+		if err != nil {
+			return renderErrorPage(c, "Error loading issues", "issues_load_error")
+		}
+		// Convert to template data
+		issues := make([]templates.ProcessingIssueData, len(dbIssues))
+		for i, issue := range dbIssues {
+			issueUUID, _ := uuid.FromBytes(issue.ID.Bytes[0:16])
+			issues[i] = templates.ProcessingIssueData{
+				ID:               issueUUID.String(),
+				MediaItemID:      uuid.UUID(issue.MediaItemID.Bytes).String(),
+				Title:            issue.Title,
+				FilePath:         issue.FilePath,
+				FormatGroup:      issue.FormatGroup,
+				LibraryTypeName:  issue.LibraryTypeName,
+				IssueType:        issue.IssueType,
+				IssueDescription: issue.IssueDescription,
+				Severity:         issue.Severity,
+				CreatedAt:        issue.CreatedAt.Time,
+			}
+		}
+		statsData := templates.IssueStats{
+			ErrorCount:   stats.ErrorCount,
+			WarningCount: stats.WarningCount,
+			InfoCount:    stats.InfoCount,
+		}
+		var buf bytes.Buffer
+		err = templates.AdminProcessingIssues(user, libraryID, issues, statsData).Render(c.Request().Context(), &buf)
+		if err != nil {
+			return err
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	}))
+
 	// Admin users page
 	frontendProtected.GET("/admin/users", handlers.AdminMiddleware(func(c *echo.Context) error {
 		user, err := getTemplateUserWithTheme(c, cfg)