chore(test): the DB lane runs on every core — each pytest-xdist worker gets its own database

backend/docker-compose.test.yml

@@ -16,6 +16,9 @@ services:
       POSTGRES_DB: siftlode_test
     tmpfs:
       - /var/lib/postgresql/data  # ephemeral + fast; the schema is rebuilt per run
+    # One database per pytest-xdist worker, each through the app's own pool (5 + 10 overflow): the default 100
+    # connections would run out at 8 workers.
+    command: ["postgres", "-c", "max_connections=300"]
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U siftlode -d siftlode_test"]
       interval: 1s
@@ -37,4 +40,4 @@ services:
       - ./:/app
     working_dir: /app
     # Coverage is on demand, not in the gate: see docs/development.md.
-    command: ["python", "-m", "pytest"]
+    command: ["python", "-m", "pytest", "-n", "auto"]