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"

site/src/components/ActivityFeed.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
   import { onMount } from 'svelte';
   import type { ActivitySummary, BASIndex, Sport } from '../lib/types';
-  import { formatDistance, formatDuration, formatElevation, formatDate, sportIcon, sportColor, sportLabel } from '../lib/format';
+  import { formatDistance, formatDuration, formatElevation, formatDate, isUnlisted, sportIcon, sportColor, sportLabel } from '../lib/format';
   import { loadIndex } from '../lib/dataloader';
 
   /** Render preview_coords as an SVG polyline path string. */
@@ -47,11 +47,11 @@
   let me: string = '';
 
   // Show private activities only to their owner.
-  // On a profile page (filterHandle set): show private if me === filterHandle.
-  // On the global feed: show private only for the logged-in user's own activities.
+  // On a profile page (filterHandle set): show unlisted if me === filterHandle.
+  // On the global feed: show unlisted only for the logged-in user's own activities.
   $: isOwner = filterHandle !== '' && me === filterHandle;
   $: withPrivacy = all.filter(a => {
-    if (a.privacy === 'private') {
+    if (isUnlisted(a.privacy)) {
       return filterHandle ? isOwner : (me !== '' && (a as any).handle === me);
     }
     return true;
@@ -161,8 +161,8 @@
             </p>
             <!-- stretched link covers the whole card; sits below the handle link -->
             <h3 class="font-semibold text-white truncate group-hover:text-[--accent] transition-colors flex items-center gap-1.5">
-              {#if a.privacy === 'private'}
-                <span class="text-zinc-500 shrink-0" title="Private">🔒</span>
+              {#if isUnlisted(a.privacy)}
+                <span class="text-zinc-500 shrink-0" title="Unlisted">🔒</span>
               {/if}
               <a
                 href={a.detail_url ? `${import.meta.env.BASE_URL}activity/${a.id}/` : `${import.meta.env.BASE_URL}activity/local/?id=${a.id}`}