feat: add target_host_id to service health checks

- Add target_host_id column to host_health_checks table (nullable UUID FK)
- Allow service checks to query a different host agent
- Backend models, API routes, and poller updated
- Frontend: host selector dropdown for service checks
- Validation: target host must exist and be healthy
- FK ON DELETE SET NULL: revert to own host if target deleted

migrations/011_health_check_target_host.sql

@@ -0,0 +1,10 @@
+-- Add target_host_id to health checks, allowing a check on Host A
+-- to query a service on Host B's agent (for redundant services).
+-- NULL = check own host (backward compatible).
+-- FK with ON DELETE SET NULL: if target host deleted, revert to default.
+
+ALTER TABLE host_health_checks
+  ADD COLUMN target_host_id UUID REFERENCES hosts(id) ON DELETE SET NULL;
+
+CREATE INDEX idx_health_checks_target_host ON host_health_checks (target_host_id)
+  WHERE target_host_id IS NOT NULL;