Add full-stack backend and deployment integration tests

[ValkyrieUK]

Summary

• Add a deterministic public Docker Compose integration stack with readsb and ACARS fixtures
• Exercise the real track service, ACARS service, TimescaleDB, production nginx image, and multi-feed configuration
• Verify ingestion, historical tracks, heatmaps, compression, retention, feature flags, proxy routing, and idempotent restart
• Add a GitHub-hosted CI workflow with no hardware, credentials, or private services required
• Fix clean database startup by passing RETENTION_DAYS to asyncpg as a timedelta interval

Coverage

The suite runs both a live-only viewer and a full historical plus ACARS multi-feed viewer. It verifies local and remote nginx routes, queries persisted data through the public APIs, inspects TimescaleDB schema and jobs, restarts both backends, and repeats all assertions.

Verification

• Python fixture and verifier syntax checks passed
• Docker Compose stack passed all assertions before backend restart
• Docker Compose stack passed all assertions again after backend restart
• Tested with Docker 29.4.0 and Docker Compose 5.1.2 on Linux