Tombstones for deleted contracts

[githoboman]

mplemented contract liveness tracking with tombstone emission on storage TTL expiry, matching the codebase's existing conventions (modeled closely on sac-detect.ts).
Closes #143

Files changed
New: src/indexer/tombstones.ts — the core module:

isExpired(liveUntilLedger, currentLedger) — pure expiry rule. A contract is live through its liveUntilLedger, so it's only expired once currentLedger > liveUntilLedger. A null/unknown TTL is never tombstoned.
tombstoneFor(liveness, currentLedger) — pure: builds a tombstone record or returns null.
fetchLiveUntilLedger — reads the contract instance entry's liveUntilLedgerSeq via getRpc().getContractData(...), exactly like SAC detection reads the executable. Wrapped in an injectable TtlFetcher type for testing.
fetchLiveness / detectExpiredContracts — de-duplicate contract IDs, one RPC call per unique contract.
insertTombstones — idempotent createMany({ skipDuplicates: true }) keyed by the unique contractId (first expiry detection wins).
tombstoneExpiredContracts — the end-to-end entry point: detect + persist.
New: src/tests/tombstones.test.ts — 15 tests over a TTL fixture (live / expired / border / unknown contracts) with an injected fetcher and a mocked Prisma client. No network, no DB.

Schema + migration — added the ContractTombstone model to schema.prisma (unique contractId, liveUntilLedger, detectedLedger, indexed by detectedLedger) plus the matching SQL migration.

Acceptance criteria
✅ Tombstones inserted on expiry detection — tombstoneExpiredContracts / insertTombstones.
✅ Tested with fixture — fixture-driven suite, 15/15 passing.
Verification
npx jest src/tests/tombstones.test.ts → 15 passed
npx tsc --noEmit (and the test tsconfig) → clean, exit 0
npx prisma generate → client regenerated for the new model

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