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 '—';

site/src/lib/types.ts

@@ -2,7 +2,9 @@
 
 export type Sport = "cycling" | "running" | "hiking" | "walking" | "swimming" | "skiing" | "other";
 export type SubSport = "road" | "mountain" | "gravel" | "indoor" | "trail" | "track" | "nordic" | "alpine" | "open_water" | "pool" | null;
-export type Privacy = "public" | "blur_start" | "no_gps" | "private";
+/** "unlisted" = not shown in the public feed; GPS track still published (security by obscurity).
+ *  "private" is the legacy alias for "unlisted" — accepted when reading old data. */
+export type Privacy = "public" | "blur_start" | "no_gps" | "unlisted" | "private";
 
 /** [duration_s, avg_watts] pairs, sorted by duration ascending. */
 export type MmpCurve = [number, number][];