feat(indexer): implement retention and pruning policy for operations and snapshots (#214)

[ntttttbl123-blip]

Closes #214

Summary of Changes

Implements a configurable retention and periodic background pruning policy for historical operations and contract snapshots in the indexer, and documents the operational bounds, storage trade-offs, and cost implications.

Key Changes

• Pruning Worker (�pps/indexer/src/workers/prune.ts): Periodically deletes \Operation\ rows older than \operationsRetentionDays\ (default: 90 days) and \ContractSnapshot\ rows older than \snapshotsRetentionDays\ (default: 30 days). Setting either to 0 retains records indefinitely.
• Config & Main Loop (�pps/indexer/src/config.ts, �pps/indexer/src/index.ts): Added \INDEXER_OPERATIONS_RETENTION_DAYS, \INDEXER_SNAPSHOTS_RETENTION_DAYS, and \INDEXER_PRUNE_INTERVAL_MS\ configuration with periodic maintenance execution and structured metrics logging.
• Policy Documentation (docs/RETENTION.md): Documented the retention rationale, default active windows, pruning cadence, and storage/cost trade-offs.
• Documentation Updates (docs/ENVIRONMENT.md, docs/INDEXER.md, .env.example): Added reference documentation for all retention variables.
• Automated Tests (�pps/indexer/src/workers/prune.test.ts): Unit test coverage for operations and snapshot pruning logic.

Verification Status

• \pnpm --filter @signet/indexer test: All 50 tests passed cleanly.
• 
  ode scripts/check-docs.mjs: Passed cleanly (23 markdown files, 30 env vars, no drift).
• 
  ode scripts/check-contract-errors.mjs: Passed.

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[blockchain-maxis]

The retention worker itself reads well — retention expressed in days with 0 meaning disabled, the cutoff computed from an injected now so the tests don't depend on the clock, and operations and snapshots given independent windows. docs/RETENTION.md plus the .env.example / docs/ENVIRONMENT.md entries are the right supporting changes, and the docs guard is happy with them.

One thing blocks it: lint • typecheck • test • build fails on this branch, and main is green.

src/workers/prune.test.ts(52,16): error TS2532: Object is possibly 'undefined'.
src/workers/prune.test.ts(72,16): error TS2532: Object is possibly 'undefined'.

Both are the array index right after the length assertion:

assert.equal(operations.length, 1);
assert.equal(operations[0].id, 'op-recent');   // line 52
...
assert.equal(snapshots.length, 1);
assert.equal(snapshots[0].id, 's-recent');     // line 72

noUncheckedIndexedAccess is on for this workspace, so operations[0] is T | undefined regardless of the assertion above it — assert.equal isn't a type guard. operations[0]!.id / snapshots[0]!.id is the fix, matching how the existing indexer tests handle it.

pnpm --filter @signet/indexer typecheck reproduces both locally. Push that and I'll merge — nothing else here needs changing.