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/sqlite/0002_create_hosts.sql

@@ -0,0 +1,9 @@
+-- hosts: physical or virtual machines belonging to a network.
+-- The ip field must fall within the CIDR of the parent network
+-- (enforced in application code, not at the DB level).
+CREATE TABLE IF NOT EXISTS hosts (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    name       TEXT    NOT NULL,
+    ip         TEXT    NOT NULL,
+    network_id INTEGER NOT NULL REFERENCES networks(id) ON DELETE CASCADE
+);