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/0003_create_ports.sql

@@ -0,0 +1,7 @@
+-- ports: global catalog of TCP/UDP port numbers and their known protocol.
+-- Ports are not tied to a specific host here — host_ports handles that link.
+-- Port numbers range from 0 to 65535 (fits in INTEGER).
+CREATE TABLE IF NOT EXISTS ports (
+    number      INTEGER PRIMARY KEY,
+    description TEXT
+);