fix(ops): rebalance container memory limits to prevent celery OOM kills

Celery worker was OOM-killed (41 restarts) at 512MB with 4 concurrent
workers. Reduce concurrency to 2, increase worker limit to 768MB, and
reclaim memory from over-provisioned services (db 512→256, beat 256→128,
flower 256→192). Total allocation stays within 4GB server budget.

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

app/core/celery_config.py

@@ -91,7 +91,7 @@ celery_app.conf.update(
     task_soft_time_limit=25 * 60,  # 25 minutes soft limit
     # Worker settings
     worker_prefetch_multiplier=1,  # Disable prefetching for long tasks
-    worker_concurrency=4,  # Number of concurrent workers
+    worker_concurrency=2,  # Keep low on 4GB servers to avoid OOM
     # Result backend
     result_expires=86400,  # Results expire after 24 hours
     # Retry policy