feat: add Atom feeds and JSON/CSV import/export

Phase 5 — Feeds & Import/Export:
- Atom feeds: global (/feed.xml), per-user (/u/{name}/feed.xml),
  per-tag (/tags/{name}/feed.xml). Uses gorilla/feeds.
- JSON export: all user's faves with tags, pretty-printed
- CSV export: standard format with header row
- JSON import: validates and creates faves with tags
- CSV import: flexible column mapping from header row
- Import/export pages with format documentation
- Feed items include enclosure for images, author info
- Limited-visibility profiles excluded from feeds

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/handler/handler.go

@@ -112,6 +112,18 @@ func (h *Handler) Routes() *http.ServeMux {
 	mux.Handle("POST /settings/avatar", requireLogin(http.HandlerFunc(h.handleAvatarPost)))
 	mux.Handle("POST /settings/password", requireLogin(http.HandlerFunc(h.handleSettingsPasswordPost)))
 
+	// Feeds (public, no auth required).
+	mux.HandleFunc("GET /feed.xml", h.handleFeedGlobal)
+	mux.HandleFunc("GET /u/{username}/feed.xml", h.handleFeedUser)
+	mux.HandleFunc("GET /tags/{name}/feed.xml", h.handleFeedTag)
+
+	// Import/Export (authenticated).
+	mux.Handle("GET /export", requireLogin(http.HandlerFunc(h.handleExportPage)))
+	mux.Handle("GET /export/json", requireLogin(http.HandlerFunc(h.handleExportJSON)))
+	mux.Handle("GET /export/csv", requireLogin(http.HandlerFunc(h.handleExportCSV)))
+	mux.Handle("GET /import", requireLogin(http.HandlerFunc(h.handleImportPage)))
+	mux.Handle("POST /import", requireLogin(http.HandlerFunc(h.handleImportPost)))
+
 	// Admin panel (requires admin role).
 	admin := func(hf http.HandlerFunc) http.Handler {
 		return requireLogin(middleware.RequireAdmin(http.HandlerFunc(hf)))