Fix/staging healthchecks

[ademboukabes]

Description

This PR prepares the infrastructure for a robust production deployment by introducing deep healthchecks and deterministic startup ordering for Docker Compose.

Key Changes

• Dynamic /health Endpoint: Replaced static response with a full liveness/readiness probe. The API now runs SELECT 1 on Postgres and PING on Redis, returning HTTP 503 if either fails.
• Native Docker Healthchecks: Added healthcheck blocks for Postgres, MinIO, Redis, NATS, and the API in docker-compose.staging.yml.
• Strict Startup (depends_on): Replaced simple dependencies with condition: service_healthy to prevent race conditions (workers wait until NATS and DB are fully ready).
• AES-256-GCM Fix: Updated encryption_key in .env.example and .env.staging.example with a valid 32-byte base64 string to fix CI test crashes caused by the invalid placeholder.
• Ops Tweaks: Removed pgadmin from staging and set migrate to restart: "no".

Validation

• Tested /health endpoint with service isolation
• All unit and integration tests pass successfully (154/154)