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/feed.go

@@ -0,0 +1,160 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package handler
+
+import (
+	"errors"
+	"log/slog"
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/gorilla/feeds"
+
+	"kode.naiv.no/olemd/favoritter/internal/model"
+	"kode.naiv.no/olemd/favoritter/internal/store"
+)
+
+const feedPageSize = 50
+
+// handleFeedGlobal generates an Atom feed of all public faves.
+func (h *Handler) handleFeedGlobal(w http.ResponseWriter, r *http.Request) {
+	baseURL := h.deps.Config.BaseURL(r.Host)
+
+	faves, _, err := h.deps.Faves.ListPublic(feedPageSize, 0)
+	if err != nil {
+		slog.Error("feed: list public error", "error", err)
+		http.Error(w, "Internal error", http.StatusInternalServerError)
+		return
+	}
+
+	if err := h.deps.Faves.LoadTags(faves); err != nil {
+		slog.Error("feed: load tags error", "error", err)
+	}
+
+	settings, _ := h.deps.Settings.Get()
+	siteName := "Favoritter"
+	if settings != nil {
+		siteName = settings.SiteName
+	}
+
+	feed := &feeds.Feed{
+		Title:       siteName + " — Siste favoritter",
+		Link:        &feeds.Link{Href: baseURL},
+		Description: "Siste offentlige favoritter",
+		Updated:     time.Now(),
+	}
+
+	feed.Items = favesToFeedItems(faves, baseURL)
+	h.writeAtom(w, feed)
+}
+
+// handleFeedUser generates an Atom feed for a user's public faves.
+func (h *Handler) handleFeedUser(w http.ResponseWriter, r *http.Request) {
+	username := r.PathValue("username")
+	baseURL := h.deps.Config.BaseURL(r.Host)
+
+	user, err := h.deps.Users.GetByUsername(username)
+	if err != nil {
+		if errors.Is(err, store.ErrUserNotFound) {
+			http.NotFound(w, r)
+			return
+		}
+		slog.Error("feed: get user error", "error", err)
+		http.Error(w, "Internal error", http.StatusInternalServerError)
+		return
+	}
+
+	if user.Disabled || user.ProfileVisibility == "limited" {
+		http.NotFound(w, r)
+		return
+	}
+
+	faves, _, err := h.deps.Faves.ListPublicByUser(user.ID, feedPageSize, 0)
+	if err != nil {
+		slog.Error("feed: list user faves error", "error", err)
+		http.Error(w, "Internal error", http.StatusInternalServerError)
+		return
+	}
+
+	if err := h.deps.Faves.LoadTags(faves); err != nil {
+		slog.Error("feed: load tags error", "error", err)
+	}
+
+	feed := &feeds.Feed{
+		Title:   user.DisplayNameOrUsername() + " sine favoritter",
+		Link:    &feeds.Link{Href: baseURL + "/u/" + user.Username},
+		Updated: time.Now(),
+	}
+
+	feed.Items = favesToFeedItems(faves, baseURL)
+	h.writeAtom(w, feed)
+}
+
+// handleFeedTag generates an Atom feed for a tag's public faves.
+func (h *Handler) handleFeedTag(w http.ResponseWriter, r *http.Request) {
+	tagName := r.PathValue("name")
+	baseURL := h.deps.Config.BaseURL(r.Host)
+
+	faves, _, err := h.deps.Faves.ListByTag(tagName, feedPageSize, 0)
+	if err != nil {
+		slog.Error("feed: list by tag error", "error", err)
+		http.Error(w, "Internal error", http.StatusInternalServerError)
+		return
+	}
+
+	if err := h.deps.Faves.LoadTags(faves); err != nil {
+		slog.Error("feed: load tags error", "error", err)
+	}
+
+	feed := &feeds.Feed{
+		Title:   "Favoritter med merkelapp: " + tagName,
+		Link:    &feeds.Link{Href: baseURL + "/tags/" + tagName},
+		Updated: time.Now(),
+	}
+
+	feed.Items = favesToFeedItems(faves, baseURL)
+	h.writeAtom(w, feed)
+}
+
+func favesToFeedItems(faves []*model.Fave, baseURL string) []*feeds.Item {
+	items := make([]*feeds.Item, 0, len(faves))
+	for _, f := range faves {
+		item := &feeds.Item{
+			Title:   f.Description,
+			Link:    &feeds.Link{Href: baseURL + "/faves/" + itoa(f.ID)},
+			Author:  &feeds.Author{Name: f.DisplayName},
+			Created: f.CreatedAt,
+			Updated: f.UpdatedAt,
+		}
+
+		if f.URL != "" {
+			item.Content = `<p><a href="` + f.URL + `">` + f.URL + `</a></p>`
+		}
+
+		if f.ImagePath != "" {
+			item.Enclosure = &feeds.Enclosure{
+				Url:  baseURL + "/uploads/" + f.ImagePath,
+				Type: "image/jpeg",
+			}
+		}
+
+		items = append(items, item)
+	}
+	return items
+}
+
+func (h *Handler) writeAtom(w http.ResponseWriter, feed *feeds.Feed) {
+	atom, err := feed.ToAtom()
+	if err != nil {
+		slog.Error("feed: generate atom error", "error", err)
+		http.Error(w, "Internal error", http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "application/atom+xml; charset=utf-8")
+	w.Write([]byte(atom))
+}
+
+func itoa(n int64) string {
+	return strconv.FormatInt(n, 10)
+}