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/grid.html

@@ -24,6 +24,7 @@
         <p class="pcount"><strong>{{.Total}}</strong> posts</p>
         <p class="pbio">{{.Bio}}{{if .BioAlt}}<br><span class="ml">{{.BioAlt}}</span>{{end}}</p>
         <a href="/" class="rss-a">Timeline</a>
+        <a href="/albums/" class="rss-a">Albums</a>
         <a href="/feed.xml" class="rss-a">
           <svg width="14" height="14" viewBox="0 0 14 14" fill="currentColor" aria-hidden="true">
             <circle cx="2.5" cy="11.5" r="1.5"/>