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.
