feat(infra): add launch readiness quick wins

- Add mem_limit to all 6 app containers (db: 512m, redis: 128m,
  api: 512m, celery-worker: 512m, celery-beat: 128m, flower: 128m)
- Restrict Flower port to localhost (127.0.0.1:5555:5555)
- Add PostgreSQL and Redis health checks to /health/ready endpoint
  with individual check details (name, status, latency)
- Add scaling guide with metrics, thresholds, Hetzner pricing
- Add server verification script (12 infrastructure checks)
- Update hetzner-server-setup.md with progress and pending tasks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/core/observability.py

@@ -531,12 +531,10 @@ async def readiness_check() -> dict[str, Any]:
     Kubernetes readiness probe endpoint.
 
     Returns 200 if the application is ready to serve traffic.
+    Includes individual check details with name, status, and latency.
     """
     result = health_registry.run_all()
-    return {
-        "status": "ready" if result.status != HealthStatus.UNHEALTHY else "not_ready",
-        "health": result.status.value,
-    }
+    return result.to_dict()
 
 
 @health_router.get("/metrics")
@@ -568,6 +566,44 @@ async def external_tools_endpoint() -> dict[str, str | None]:
 # =============================================================================
 
 
+def _register_infrastructure_health_checks() -> None:
+    """Register health checks for core infrastructure (PostgreSQL, Redis)."""
+    from .config import settings
+
+    @health_registry.register("database")
+    def check_database() -> HealthCheckResult:
+        try:
+            from .database import engine
+
+            with engine.connect() as conn:
+                from sqlalchemy import text
+
+                conn.execute(text("SELECT 1"))
+            return HealthCheckResult(name="database", status=HealthStatus.HEALTHY)
+        except Exception as e:
+            return HealthCheckResult(
+                name="database",
+                status=HealthStatus.UNHEALTHY,
+                message=str(e),
+            )
+
+    @health_registry.register("redis")
+    def check_redis() -> HealthCheckResult:
+        try:
+            import redis
+
+            r = redis.from_url(settings.redis_url, socket_connect_timeout=2)
+            r.ping()
+            r.close()
+            return HealthCheckResult(name="redis", status=HealthStatus.HEALTHY)
+        except Exception as e:
+            return HealthCheckResult(
+                name="redis",
+                status=HealthStatus.UNHEALTHY,
+                message=str(e),
+            )
+
+
 def init_observability(
     enable_metrics: bool = False,
     sentry_dsn: str | None = None,
@@ -587,6 +623,9 @@ def init_observability(
     """
     logger.info("Initializing observability stack...")
 
+    # Register infrastructure health checks
+    _register_infrastructure_health_checks()
+
     # Enable metrics if requested
     if enable_metrics:
         metrics_registry.enable()