fix(training-agent): fall back to InMemoryReplayStore when no DB initialized

[bokelley]

Closes #3376

Summary

PR #3351 swapped InMemoryReplayStore for PostgresReplayStore to fix cross-instance replay detection on Fly's multi-machine deployment. The storyboard CI runner (run-storyboards.ts) starts the training-agent without Postgres — no DATABASE_URL, no initializeDatabase() call — so getPool() threw "Database not initialized" on every signed request. All 28 positive signed_requests storyboard vectors returned 401 instead of 2xx; every PR against main has been failing CI since 2bb3dcd8.

Fix: check isDatabaseInitialized() before choosing the store in buildAuthenticatorWithCapability():

• Production (DB always initialized before listen() per http.ts:8531 vs 8641): unchanged — shared PostgresReplayStore singleton, cross-instance replay protection.
• CI / storyboard runner (no DB): per-authenticator InMemoryReplayStore, restoring exact pre-fix(training-agent): PostgresReplayStore so vector 016 catches cross-instance replays #3351 behavior. Each route gets its own store, so cross-route false positives can't occur (training-agent verifier: replay store doesn't reject duplicate (keyid, nonce) — fails grader neg/016 #3338).

Also guards startReplayCacheSweeper()'s interval callback with if (!isDatabaseInitialized()) return to prevent tick-level getPool() throws if the sweeper is ever started in a no-DB context.

Note on neg/016 in CI: The per-authenticator InMemoryReplayStore correctly handles the intra-run replay vector via white-box harness injection (documented in negative/016-replayed-nonce.json). Cross-instance replay (the scenario #3351 was solving for) is only validated in production against the Postgres-backed verifier — CI tests the single-process guarantee only.

Non-breaking justification

Server-side infrastructure change only. No schema fields, task definitions, or @adcp/* package API changed. In production the Postgres store path is unaffected. Changeset: --empty.

Pre-PR review

• code-reviewer: approved — no blockers; fixed stale JSDoc on buildStrictRequestSigningAuthenticator per reviewer's issue flag; nits surfaced (missing unit test for isDatabaseInitialized() branch, sweeper guard is defensive dead-code in current call sites)
• ad-tech-protocol-expert: approved — isDatabaseInitialized() guard is safe per boot sequence; non-breaking per spec definition; changeset classification correct; neg/016 CI scope documented

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See #3121
for context.

Session: https://claude.ai/code/session_015vh3dBv4iAKcoWj4a16ygU

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[bokelley]

Routine: persistent DIRTY against main. Please rebase locally, resolve, and force-push.

[bokelley]

Closing as superseded — #3379 landed the same fix on main (commit c614493b0). CI green since.