rename privacy "private" → "unlisted"; enable GPS for unlisted

- "unlisted" = not shown in the public feed, but GPS track, timeseries
  and detail JSON are all accessible by direct URL (security by obscurity)
- "private" accepted as legacy alias everywhere (backward compat with
  existing data on disk)
- New writes from Strava sync / ZIP upload / sidecar use "unlisted"
- Only "no_gps" now suppresses the GPS track
- isUnlisted() helper in format.ts used by all Svelte/Astro components
- SCHEMA.md and CLAUDE.md document the privacy model and the distinction
  between "unlisted" and "no_gps"

bincio/edit/server.py

@@ -183,7 +183,7 @@ textarea { resize: vertical; min-height: 140px; }
             <input type="checkbox" id="highlight" name="highlight"> Highlight in feed
           </label>
           <label class="toggle" id="toggle-private">
-            <input type="checkbox" id="private" name="private"> Private (hide from feed)
+            <input type="checkbox" id="private" name="private"> Unlisted (hide from feed)
           </label>
         </div>
       </div>