feat(host_agent): agent reports its primary IP; mirror into Host.address (task 274)

The agent now detects its own primary non-loopback IP (UDP-socket trick, no
packets sent) and sends it in the metadata bag; bumps AGENT_VERSION 1.0.0->1.1.0
and re-detects on SIGHUP. The server persists it on HostAgentRegistration.host_ip
and mirrors it into Host.address ONLY when that field is blank — an admin-typed
address is never overwritten. The reported IP shows in the settings table so
admins can see drift regardless.

- agent.py: detect_primary_ip(); host_ip in collect_metadata(); refresh on reload
- routes.py: pure pick_host_address() helper; ingest persists host_ip + mirrors
- models.py + migration host_agent_002_host_ip: new String(45) column
- settings_list.html: Reported IP column
- plugin.yaml: 1.0.0 -> 1.1.0
- tests: pick_host_address (fill-when-blank / never-clobber / no-op),
  detect_primary_ip (never raises, non-loopback), metadata carries host_ip

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

plugins/host_agent/routes.py

@@ -34,6 +34,19 @@ def _parse_ts(ts: str) -> datetime:
     return datetime.fromisoformat(ts)
 
 
+def pick_host_address(current: str | None, reported: str | None) -> str | None:
+    """Return the address to store on the Host, or None for no change.
+
+    The agent-reported IP fills Host.address only when the current value is
+    blank — an admin-typed address (DNS name, management IP) is never
+    overwritten. The reported IP is always kept on the registration regardless,
+    so admins can still see drift.
+    """
+    if reported and not (current or "").strip():
+        return reported
+    return None
+
+
 def _expand_sample_to_metrics(
     sample: dict, host_name: str, recorded_at: datetime
 ) -> list[PluginMetric]:
@@ -157,6 +170,14 @@ async def ingest():
             if field in md and getattr(reg, field) != md[field]:
                 setattr(reg, field, md[field])
                 changed = True
+        host_ip = md.get("host_ip")
+        if host_ip and reg.host_ip != host_ip:
+            reg.host_ip = host_ip
+            changed = True
+        # Mirror the reported IP into Host.address only when it's blank.
+        new_address = pick_host_address(host.address, host_ip)
+        if new_address is not None:
+            host.address = new_address
         if changed:
             reg.updated_at = datetime.now(timezone.utc)