Go global: worldwide BTC Map sync, marker clustering, response compression

- Sync now pulls every BTC Map place worldwide (~29k live), not just
  Madeira — dropped BTCMAP_CENTER_LAT/LON/RADIUS_KM entirely.
- Incremental via a stored watermark (settings.btcmap_synced_since):
  only fetches what changed since the last sync, not the whole dataset
  every time. Handles deletions too (BTC Map delistings correctly evict
  the local place + cascade its claim/links, not just go unclaimed).
- Real bug caught and fixed before it shipped: tried BTC Map's own
  recommended updated_since+limit pagination first, but their timestamps
  mix millisecond and whole-second precision, and naive string comparison
  across that isn't chronologically safe — silently truncated a real
  sync to ~4,000 of ~42,000 records with no error. Measured the
  alternative (single unpaginated request) instead: 2-3s for the full
  dataset, simpler and actually correct. Full writeup in
  services/btcmap.ts, regression test for the specific bug in
  services/btcmap.test.ts.
- @fastify/compress added — the places list is now tens of thousands of
  rows, gzip/br/zstd auto-negotiated.
- Also fixed while touching dependencies: @fastify/static had a real
  high-severity path-traversal/auth-bypass advisory (GHSA-pr96-94w5-mx2h
  et al) affecting the version we were pinned to — bumped to the patched
  10.1.2.
- Frontend: leaflet.markercluster (raw per-marker rendering doesn't scale
  to tens of thousands of points), cyberpunk-themed cluster icons to
  match the existing HUD styling, batch marker insertion (addLayers, not
  a per-marker addLayer loop) since that's dramatically faster at this
  scale. Default map view is now world-scale, recentering on the
  player's location if geolocation is available.
- 60 backend tests passing throughout (7 new for the sync rewrite).

frontend/src/style.css

@@ -83,3 +83,21 @@ h1, h2, .font-display {
 .regress-link-green { filter: drop-shadow(0 0 4px #39ff14); }
 .regress-field-orange { filter: drop-shadow(0 0 6px #ff5f1f); }
 .regress-field-green { filter: drop-shadow(0 0 6px #39ff14); }
+
+/* Reskin leaflet.markercluster's default light-green bubble look to match
+   the HUD theme — overrides MarkerCluster.Default.css. */
+.regress-cluster div {
+  width: 100%;
+  height: 100%;
+  border-radius: 50%;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: radial-gradient(circle at 35% 35%, rgba(0, 255, 242, 0.35), rgba(0, 255, 242, 0.12) 70%);
+  border: 2px solid #00fff2;
+  color: #d8fbff;
+  font-family: 'Share Tech Mono', monospace;
+  font-weight: bold;
+  font-size: 12px;
+  box-shadow: 0 0 10px rgba(0, 255, 242, 0.5);
+}