v0.22.1: dockerize defaults, harden error handling, fix test discovery

- config.py: defaults to localhost/localhost, AMQP to rabbitmq, secret
  path to /run/secrets, RAG_MAX_VECTORS from env
- rag.py: EMBED_URL default to localhost
- app.py: syslog handler wrapped in try/except
- routers/completions.py: db.close() in try/finally
- db.py: PRAGMA journal_mode = WAL
- amqp.py: subscription append before try for reconnect safety
- tests/conftest.py: add project root to sys.path for test discovery

tests/conftest.py

@@ -0,0 +1,6 @@
+# Ensure the project root is on sys.path so that test modules can import
+# top-level packages (app, amqp, cluster, config, …) without PYTHONPATH hacks.
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))