feat(plugins): default-enable generic bundled plugins on fresh install

A fresh install enabled zero plugins — settings.py DEFAULTS had no plugin.*
keys, so to_plugins_cfg returned {} and every plugin had to be flipped on
by hand. Seed docker, host_agent, http, snmp as default-on (generic, non-
vendor-specific); traefik and unifi stay opt-in. Stored choices override
the default, so disabling persists.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/core/settings.py

@@ -60,6 +60,17 @@ DEFAULTS: dict[str, Any] = {
     "ping.threshold.good_ms": 50,
     "ping.threshold.warn_ms": 200,
     "plugins.index_url": "https://git.fabledsword.com/bvandeusen/Steward-plugins/raw/branch/main/index.yaml",
+    # Default-enabled plugins. These are the generic, non-vendor-specific
+    # bundled plugins (protocols/standards, not a single product) — useful on
+    # almost any install, so a fresh deployment comes up monitoring rather than
+    # blank. Vendor-specific plugins (traefik, unifi) stay opt-in. An operator
+    # who disables one writes plugin.<name>={"enabled": False}, which overrides
+    # these defaults (stored value wins in get_all_settings/load_settings_sync).
+    # Per-plugin yaml config defaults are merged on top at load time.
+    "plugin.docker": {"enabled": True},
+    "plugin.host_agent": {"enabled": True},
+    "plugin.http": {"enabled": True},
+    "plugin.snmp": {"enabled": True},
     # OIDC single-sign-on
     "oidc.enabled": False,
     "oidc.discovery_url": "",