feat(host-detail): add direct host-application association with modal multi-select

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

migrations/postgres/0008_create_host_applications.sql

@@ -0,0 +1,8 @@
+-- host_applications: direct association between a host and an application.
+-- Allows explicitly tagging a host with an application regardless of ports.
+-- One application can only be linked once to a given host.
+CREATE TABLE IF NOT EXISTS host_applications (
+    host_id        BIGINT NOT NULL REFERENCES hosts(id)        ON DELETE CASCADE,
+    application_id BIGINT NOT NULL REFERENCES applications(id) ON DELETE CASCADE,
+    PRIMARY KEY (host_id, application_id)
+);