test/infra: pg-pool DATABASE_URL test, event wire-format guard, docker/render hardening

[Johnalex-hub]

Summary

Closes #1055
Closes #1056
Closes #1057
Closes #1058

• [Testing] backend/src/lib/pg-pool.ts has no test verifying the pool is created with the configured DATABASE_URL #1055: Added backend/tests/pg-pool.test.ts asserting createPgPoolConfig/createPgPool pick up an overridden DATABASE_URL, with pg.Pool mocked so no real connection is opened.
• [Testing] contracts/stream_contract/src/events.rs has no test asserting event field ordering/types match the indexer's decoder expectations #1056: Added a contract-side regression test (test_stream_created_event_field_names_match_decoder_expectations in contracts/stream_contract/src/test.rs) that captures the raw emitted Map field names for stream_created and asserts they match what soroban-event-worker.ts's decodeMap expects. Added a matching backend fixture (backend/tests/events-wire-format.test.ts) pinning the field set for every event type the worker decodes. Documented the wire-format contract in events.rs doc comments.
• [Infra] docker-compose.yml does not pin the Postgres image to a specific minor version #1057: docker-compose.yml's Postgres service was already pinned to an immutable digest (postgres:16-alpine@sha256:...); added a comment documenting the pin and the upgrade policy.
• [Infra] render.yaml does not set a health check path for the backend service #1058: render.yaml already sets healthCheckPath: /health against the existing backend/src/routes/health.routes.ts endpoint; verified it's correctly wired, no code change needed.

Test plan

• cargo test in contracts/stream_contract — 106 passed
• npx vitest run tests/pg-pool.test.ts — 2 passed
• npx vitest run tests/events-wire-format.test.ts — 9 passed
• docker compose config -q validates docker-compose.yml