feat(seo): add robots.txt, sitemap, and noindex tags for downloads

Register routes for robots.txt and sitemap.xml, and implement search engine indexing controls to protect user privacy.

Specifically:
- Set `X-Robots-Tag: noindex, nofollow, noarchive` headers on file downloads, thumbnails, and zip generation.
- Configure `Robots: web.RobotsNone` on download and preview pages to prevent indexing of temporary user uploads.
- Add canonical URLs, improved descriptions, and image alt tags to page metadata for better social sharing.

backend/libs/web/renderer.go

@@ -7,6 +7,9 @@ import (
 	"time"
 )
 
+// RobotsNone is used for private, protected, expired, or temporary pages.
+const RobotsNone = "noindex,nofollow,noarchive"
+
 type Renderer struct {
 	templates  map[string]*template.Template
 	appName    string
@@ -15,16 +18,19 @@ type Renderer struct {
 }
 
 type PageData struct {
-	AppName     string
-	AppVersion  string
-	BaseURL     string
-	Title       string
-	Description string
-	ImageURL    string
-	CurrentYear int
-	CurrentUser any
-	CSRFToken   string
-	Data        any
+	AppName      string
+	AppVersion   string
+	BaseURL      string
+	CanonicalURL string
+	Robots       string
+	Title        string
+	Description  string
+	ImageURL     string
+	ImageAlt     string
+	CurrentYear  int
+	CurrentUser  any
+	CSRFToken    string
+	Data         any
 }
 
 func NewRenderer(templateDir, appName, appVersion, baseURL string) (*Renderer, error) {