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.

generator.go

@@ -41,8 +41,9 @@ var ogQualityLadder = []int{85, 78, 70, 62}
 
 // PhotoMeta is the structure of a .toml sidecar file.
 type PhotoMeta struct {
-	Caption string `toml:"caption"`
-	Date    string `toml:"date"` // YYYY-MM-DD
+	Caption string   `toml:"caption"`
+	Date    string   `toml:"date"`           // YYYY-MM-DD
+	Tags    []string `toml:"tags,omitempty"` // free-text, lowercased; each tag is an album
 }
 
 // Photo is the in-memory representation of one image.
@@ -56,6 +57,7 @@ type Photo struct {
 	OGHeight int
 	Caption  string
 	Date     time.Time
+	Tags     []Tag // albums this photo belongs to
 }
 
 // DayGroup is a run of photos sharing the same calendar day, for the timeline.
@@ -121,7 +123,7 @@ func (g *Generator) Build() error {
 		return err
 	}
 
-	for _, d := range []string{"images", "thumbs", "medium", "og", "photo", "grid", "day", "static"} {
+	for _, d := range []string{"images", "thumbs", "medium", "og", "photo", "grid", "day", "album", "albums", "static"} {
 		if err := os.MkdirAll(filepath.Join(g.outputDir, d), 0755); err != nil {
 			return err
 		}
@@ -142,6 +144,16 @@ func (g *Generator) Build() error {
 	if err := g.renderDayPages(photos); err != nil {
 		return err
 	}
+	albums := groupByTag(photos)
+	if err := g.renderAlbumPages(albums, len(photos)); err != nil {
+		return err
+	}
+	if err := g.renderAlbumsIndex(albums, len(photos)); err != nil {
+		return err
+	}
+	if err := g.renderTagsJSON(albums); err != nil {
+		return err
+	}
 	for i := range photos {
 		var newer, older *Photo
 		if i > 0 {
@@ -209,6 +221,7 @@ func (g *Generator) loadPhotos() ([]Photo, error) {
 			OG:      "og/" + slug + ".jpg",
 			Caption: meta.Caption,
 			Date:    date,
+			Tags:    parseTags(meta.Tags...),
 		})
 	}