explore: shard tracks into per-year files for progressive loading

bake_tracks now writes tracks_YYYY.json shards + tracks_index.json manifest
instead of a single monolithic tracks.json. API /api/me/tracks returns the
manifest; /api/me/tracks/{year} serves individual shards. Explore.svelte
fetches the two most recent years eagerly then streams the rest in the
background so the map renders immediately with recent data.

bincio/serve/routers/me.py

@@ -43,23 +43,39 @@ def _wipe_user_activities(user_dir: Path) -> int:
         if d.exists():
             shutil.rmtree(d)
 
-    for name in ("index.json", "athlete.json", ".bincio_cache.json", "tracks.json"):
+    for name in ("index.json", "athlete.json", ".bincio_cache.json", "tracks.json", "tracks_index.json"):
         f = user_dir / name
         if f.exists():
             f.unlink()
             deleted += 1
 
+    for shard in user_dir.glob("tracks_*.json"):
+        shard.unlink(missing_ok=True)
+        deleted += 1
+
     return deleted
 
 
 @router.get("/api/me/tracks")
 async def me_tracks(bincio_session: str | None = Cookie(default=None)) -> Response:
-    """Return the pre-baked tracks.json for the logged-in user (Explore page)."""
+    """Return the tracks manifest (years list + total) for the logged-in user."""
     user = deps._require_user(bincio_session)
-    tracks_path = deps._get_data_dir() / user.handle / "tracks.json"
-    if not tracks_path.exists():
+    index_path = deps._get_data_dir() / user.handle / "tracks_index.json"
+    if not index_path.exists():
         raise HTTPException(404, "Tracks not yet baked — upload an activity first")
-    return Response(content=tracks_path.read_bytes(), media_type="application/json")
+    return Response(content=index_path.read_bytes(), media_type="application/json")
+
+
+@router.get("/api/me/tracks/{year}")
+async def me_tracks_year(year: str, bincio_session: str | None = Cookie(default=None)) -> Response:
+    """Return the pre-baked tracks shard for a specific year."""
+    user = deps._require_user(bincio_session)
+    if not year.isdigit() or len(year) != 4:
+        raise HTTPException(400, "year must be a 4-digit string")
+    shard_path = deps._get_data_dir() / user.handle / f"tracks_{year}.json"
+    if not shard_path.exists():
+        raise HTTPException(404, f"No tracks shard for year {year}")
+    return Response(content=shard_path.read_bytes(), media_type="application/json")
 
 
 @router.get("/api/me/storage")