Add 500km link cap, block-height-based claim expiry, and link count in score

- MAX_LINK_DISTANCE_KM (default 500): flat cap on link length. Real
  Ingress scales this off portal/resonator level (up to ~655km); Regress
  has no leveling system to scale off of, so this is a single ceiling.
- PORTAL_TIMEOUT_BLOCKS (default 2100): claims stamp the current Bitcoin
  block height (claimed_at_block, new migration) and expire back to
  neutral after this many blocks — the game's clock is block height, not
  wall-clock. Current height comes from mempool.space (BlockHeightService,
  60s cache), same trust model as the BTC Map dependency. A background
  sweep actively reverts timed-out claims (tearing down their links, same
  as a normal recapture); claim/link routes also check inline so
  correctness doesn't depend on sweep timing between runs.
- Pre-existing claims (already live in production) get backfilled with
  the real current block height once at startup rather than a guessed
  historical value — dry-run verified against an actual copy of the live
  production database before deploying.
- Score now reports link count per team alongside places/fields/area
  (backend already computed this; only the frontend display was missing).

22 new tests (53 total): pure expiry-logic unit tests plus full
claim/link/sweep integration tests using a network-free fake block-height
provider (BlockHeightProvider interface + FakeBlockHeight test double).

frontend/src/views/MapView.vue

@@ -182,11 +182,13 @@ watch(linkSourceId, redraw);
         <span class="font-display text-lg font-black tracking-widest text-cyan-neon text-glow-cyan">REGRESS</span>
         <span class="flex items-center gap-1 font-bold text-orange-neon text-glow-orange">
           ▲ {{ orangeScore?.claimedPlaces ?? 0 }} <span class="text-orange-neon/60">nodes</span> ·
+          {{ orangeScore?.links ?? 0 }} <span class="text-orange-neon/60">links</span> ·
           {{ orangeScore?.fields ?? 0 }} <span class="text-orange-neon/60">fields</span> ·
           {{ (orangeScore?.areaKm2 ?? 0).toFixed(2) }} km²
         </span>
         <span class="flex items-center gap-1 font-bold text-green-neon text-glow-green">
           ▲ {{ greenScore?.claimedPlaces ?? 0 }} <span class="text-green-neon/60">nodes</span> ·
+          {{ greenScore?.links ?? 0 }} <span class="text-green-neon/60">links</span> ·
           {{ greenScore?.fields ?? 0 }} <span class="text-green-neon/60">fields</span> ·
           {{ (greenScore?.areaKm2 ?? 0).toFixed(2) }} km²
         </span>