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).

README.md

@@ -42,14 +42,27 @@ Map's public API.
   (default 40m) of it, via browser geolocation — no claiming from the couch.
 - Claiming an enemy-held place captures it for your team and tears down any
   links running through it.
-- A link can be created between any two places your team currently holds,
-  as long as you're standing at the origin place — no maximum link distance.
+- A link can be created between any two places your team currently holds, as
+  long as you're standing at the origin place, up to `MAX_LINK_DISTANCE_KM`
+  (default 500km) apart. Ingress scales its link range off portal/resonator
+  level (up to ~655km at max level); Regress has no leveling system to scale
+  off of, so this is a single flat cap instead.
 - A new link can't cross any existing link (either team) — same as Ingress.
   Links sharing an endpoint don't count as crossing. See
   `services/geo.ts#segmentsIntersect` and `routes/links.ts`.
 - Three mutually-linked, same-team places form a field (computed live, not
   stored) — see `server/src/services/fields.ts`.
 - Team choice is one-way once made.
+- **Claims expire after `PORTAL_TIMEOUT_BLOCKS` Bitcoin blocks (default
+  2100)** and revert to neutral, tearing down any links through them — the
+  game's clock is block height, not wall-clock time. Current height comes
+  from a public block explorer API (`BLOCK_HEIGHT_API_URL`, defaults to
+  mempool.space), same trust model as pulling place data from BTC Map. A
+  background sweep (`EXPIRY_SWEEP_INTERVAL_MS`, default 5 min) actively
+  reverts timed-out claims; claim/link requests also check inline so
+  correctness doesn't depend on sweep timing (`services/expiry.ts`).
+- Score (`GET /api/score`) reports claimed places, **link count**, field
+  count, and total field area per team.
 
 ## Tests