ci(coverage): add postgres + redis + mongo service containers

[mastermanas805]

Summary

• Worker's coverage workflow was running go test ./... against a runner with no DB / no Redis / no Mongo. Integration-style tests (the propagation_runner live-DB SKIP LOCKED walk, the cross-repo audit-kind registry walk) all silently skipped, so coverage reflected only the pure-unit slice.
• Mirrors api PR #135 + worker ci.yml's proven-green services pattern: postgres:16-alpine, redis:7-alpine, mongo:6 with health checks.
• Wires the env vars worker tests / config actually read: TEST_DATABASE_URL, TEST_REDIS_URL, TEST_POSTGRES_CUSTOMERS_URL, TEST_MONGO_URI, plus INSTANT_API_REPO for the CLAUDE.md rule-18 cross-repo gate.
• Applies api-owned migrations first (canonical platform_db schema lives in api/internal/db/migrations/), then worker-owned worker/sql/*.sql on top — matches what make test-db-up does for an integrated test run.
• Checks out api sibling at ../api so coverage_registry_test.go::findApiRepoRoot resolves and the audit-kind drift detection participates in coverage.
• Test step is continue-on-error: true so a single flaky test never drops the coverage artifact — codecov still ingests coverage.out.

Per CLAUDE.md rule 23 (the local gate must equal CI) — coverage should run the same hermetic suite ci.yml does.

Test plan

• CI runs green (or at least produces a real coverage.out)
• Codecov upload reports a realistic coverage % (not the prior no-DB slice)
• No regression in ci.yml's separate build-and-test job

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

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