chore: remove config.yaml mount from docker-compose; update config example

- Replace individual volume mounts (playbook_cache, plugins) with single app_data:/data mount
- Change FABLEDNETMON_DATABASE__URL to FABLEDNETMON_DATABASE_URL (underscore to hyphen)
- Simplify config.example.yaml to focus on required database URL setting
- Document that other settings are now managed via the Settings UI

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

config.example.yaml

@@ -1,36 +1,21 @@
-# FabledNetMon configuration example
-# Copy to config.yaml and fill in your values.
-# Secrets can be set via env vars: FABLEDNETMON_<UPPER__PATH>
-
-secret_key: "change-me-to-a-random-string"
+# FabledNetMon — Bootstrap Configuration Example
+#
+# The only REQUIRED setting is the database URL.
+# Set it via env var (recommended for Docker):
+#
+#   FABLEDNETMON_DATABASE_URL=postgresql+asyncpg://user:pass@host/db
+#
+# All other settings (SMTP, webhook, ansible, plugins, retention)
+# are stored in the database and managed via the Settings UI at /settings/.
+#
+# config.yaml is optional. Use it only if you prefer file-based bootstrap.
 
 database:
   url: "postgresql+asyncpg://fablednetmon:password@localhost/fablednetmon"
 
-session:
-  lifetime_hours: 8
+# Optional: override the auto-generated secret key.
+# If not set, a key is auto-generated on first run and saved to /data/secret.key.
+# secret_key: "change-me-to-a-random-string"
 
-data:
-  retention_days: 90
-
-# plugin_dir: ./plugins   # optional; defaults to ./plugins relative to config.yaml
-
-plugins: {}
-  # traefik:
-  #   enabled: true
-  #   metrics_url: "http://localhost:8080/metrics"
-  #   scrape_interval_seconds: 60
-
-smtp:
-  host: ""
-  port: 587
-  tls: true
-  username: ""
-  password: ""        # or: FABLEDNETMON_SMTP__PASSWORD env var
-  recipients: []
-  #   - admin@example.com
-
-webhook:
-  url: ""             # leave empty to disable
-  template: |
-    {"content": "**{{ alert.state }}** — {{ alert.resource }} — {{ alert.rule_name }} ({{ alert.metric }} = {{ alert.value }})"}
+# Optional: path to plugin directory (default: "plugins")
+# plugin_dir: "./plugins"