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/lib/format.ts

@@ -1,4 +1,10 @@
-import type { Sport } from './types';
+import type { Privacy, Sport } from './types';
+
+/** True for "unlisted" activities (and the legacy "private" alias).
+ *  Use this everywhere instead of comparing against 'private' directly. */
+export function isUnlisted(privacy: Privacy | string | null | undefined): boolean {
+  return privacy === 'unlisted' || privacy === 'private';
+}
 
 export function formatDistance(m: number | null, unit: 'metric' | 'imperial' = 'metric'): string {
   if (m == null) return '—';