Add Arrtrix runtime, config, onboarding, and webhook support

- Implement runtime package for bridge startup, config loading, and env
  overrides
- Add onboarding package for management room welcome messages
- Add matrixcmd package for command processing and help
- Add webhook package with Radarr webhook support and validation
- Extend connector config for webhooks and validation
- Update default config and example config for new options
- Add tests for new packages and config validation
- Change database type default to sqlite3-fk-wal

packages/arrtrix/pkg/connector/example-config.yaml

@@ -1,7 +1,10 @@
-# No network-specific config is required yet.
+# Arrtrix-specific runtime options.
 #
-# Future Arr-specific runtime options, such as webhook handling, can be added
-# here without changing the shared mautrix bridge CLI/runtime shape.
-#
-# The CLI-provided config file is still fully used by the bridge runtime for
-# all shared sections like bridge, database, homeserver, and appservice.
+webhooks:
+  radarr:
+    enabled: false
+    path: /_arrtrix/webhooks/radarr
+    secret: ""
+    # The first implementation delivers notifications to the only configured
+    # management room. If more than one management room exists, the webhook is
+    # rejected until routing is configured more explicitly.