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.

templates/photo.html

@@ -58,6 +58,11 @@
         </div>
         <p class="dcaption">{{.Photo.Caption}}</p>
         <p class="ddate">{{.Photo.Date.Format "Jan 02, 2006"}}</p>
+        {{if .Photo.Tags}}
+        <div class="dtags">
+          {{range .Photo.Tags}}<a href="/album/{{.Slug}}/" class="tag">{{.Name}}</a>{{end}}
+        </div>
+        {{end}}
         <p class="dfull"><a href="/{{.Photo.File}}" download>↓ Full size</a></p>
       </div>
     </div>