feat(repository): add CRUD layer for all domain entities

- Add server/repository/ module with networks, hosts, ports, applications
- Use sqlx::query() + manual row mapping (no compile-time DB required)
- Handle unique-constraint conflicts with is_unique_violation() for
  cross-database compatibility (SQLite + PostgreSQL via AnyPool)
- add_port_to_host auto-registers the port in the catalog (prevents FK errors)
- application_ports has no FK to ports (intentional: loose association)
- Add DbError::NotFound variant for missing-record cases

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

src/server/db.rs

@@ -21,6 +21,9 @@ pub enum DbError {
 
     #[error("Migration failed: {0}")]
     Migration(#[from] sqlx::migrate::MigrateError),
+
+    #[error("Record not found: {0}")]
+    NotFound(String),
 }
 
 // ─── Pool creation ────────────────────────────────────────────────────────────