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/upload.html

@@ -161,6 +161,11 @@
       <input type="date" id="date">
     </div>
 
+    <div class="field">
+      <label for="tags">Tags</label>
+      <input type="text" id="tags" placeholder="comma-separated, lowercase" autocomplete="off">
+    </div>
+
     <button class="submit-btn" id="submit-btn" disabled>Upload photo</button>
     <div id="feedback"></div>
   </div>
@@ -256,6 +261,7 @@
       form.append('image',   file);
       form.append('caption', document.getElementById('caption').value);
       form.append('date',    dateInput.value);
+      form.append('tags',    document.getElementById('tags').value);
 
       try {
         const res  = await fetch('/upload', { method: 'POST', body: form });
@@ -268,6 +274,7 @@
           prompt.style.display  = 'flex';
           dropZone.classList.remove('has-image');
           document.getElementById('caption').value = '';
+          document.getElementById('tags').value = '';
           submitBtn.disabled = true;
         } else {
           feedback.className   = 'err';