Add albums: group photos by free-text tags

An album is a tag. Photos carry lowercase free-text tags in their .toml
sidecar (`tags = [...]`), parallel to the existing day grouping — no
database, all derived at build time.

- albums.go: parseTags (lowercase/trim/dedupe, slug via sanitiseSlug),
  groupByTag mirroring groupByDay, and rendering for album pages, the
  index, and the JSON API. Slug collisions between distinct names are
  suffixed and logged.
- Routes: /album/<slug>/ (paginated square grid per tag), /albums/ (index
  with counts), /tags.json (machine-readable tag list, served statically
  like feed.xml).
- POST /upload gains a `tags` field written into the sidecar; the web
  upload form gains a tags input.
- Tags shown on each photo page linking to their album; an Albums link in
  the timeline/grid header.

server.go

@@ -33,7 +33,7 @@ func NewServer(port, contentDir, outputDir string, serve bool, g *Generator) *Se
 
 func (s *Server) Listen() error {
 	mux := http.NewServeMux()
-	mux.HandleFunc("GET /upload",  s.handleUploadPage)
+	mux.HandleFunc("GET /upload", s.handleUploadPage)
 	mux.HandleFunc("POST /upload", s.handleUpload)
 	mux.HandleFunc("GET /health", func(w http.ResponseWriter, r *http.Request) {
 		fmt.Fprintln(w, "ok")
@@ -67,6 +67,7 @@ func (s *Server) handleUploadPage(w http.ResponseWriter, r *http.Request) {
 //	image    — the image file (jpg or png)
 //	caption  — human-readable caption (optional, defaults to filename)
 //	date     — YYYY-MM-DD (optional, defaults to today)
+//	tags     — comma-separated album tags (optional; lowercased, deduped)
 func (s *Server) handleUpload(w http.ResponseWriter, r *http.Request) {
 	if err := r.ParseMultipartForm(32 << 20); err != nil {
 		http.Error(w, "bad request", http.StatusBadRequest)
@@ -95,11 +96,18 @@ func (s *Server) handleUpload(w http.ResponseWriter, r *http.Request) {
 		caption = strings.TrimSuffix(header.Filename, ext)
 	}
 
+	// Normalize the tags once here so the sidecar stores clean lowercase names;
+	// the generator derives album slugs from them at build time.
+	var tags []string
+	for _, t := range parseTags(r.FormValue("tags")) {
+		tags = append(tags, t.Name)
+	}
+
 	slug := sanitiseSlug(strings.TrimSuffix(header.Filename, ext))
 	if slug == "" {
 		slug = fmt.Sprintf("photo-%d", time.Now().UnixMilli())
 	}
-	imgPath  := filepath.Join(s.contentDir, slug+ext)
+	imgPath := filepath.Join(s.contentDir, slug+ext)
 	tomlPath := filepath.Join(s.contentDir, slug+".toml")
 
 	out, err := os.Create(imgPath)
@@ -121,7 +129,7 @@ func (s *Server) handleUpload(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	defer tf.Close()
-	if err := toml.NewEncoder(tf).Encode(PhotoMeta{Caption: caption, Date: dateStr}); err != nil {
+	if err := toml.NewEncoder(tf).Encode(PhotoMeta{Caption: caption, Date: dateStr, Tags: tags}); err != nil {
 		http.Error(w, "could not write metadata", http.StatusInternalServerError)
 		return
 	}