feat(db): add SQLx migrations and AppState with connection pool

- Add sqlx 0.8 (AnyPool, runtime-tokio, sqlite, postgres, migrate)
- Create 6 migration files for both SQLite and PostgreSQL backends
- Add server/db.rs: create_pool and run_migrations helpers
- Add server/state.rs: AppState with LeptosOptions + AnyPool
- Run migrations at server startup before accepting requests
- Fix Port model: remove host_id (ports are now a global catalog)
- Add HostPort join struct for the host_ports many-to-many table

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

migrations/postgres/0005_create_host_ports.sql

@@ -0,0 +1,6 @@
+-- host_ports: which ports are open on which host (many-to-many).
+CREATE TABLE IF NOT EXISTS host_ports (
+    host_id     BIGINT  NOT NULL REFERENCES hosts(id) ON DELETE CASCADE,
+    port_number INTEGER NOT NULL REFERENCES ports(number) ON DELETE CASCADE,
+    PRIMARY KEY (host_id, port_number)
+);