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/repository/mod.rs

@@ -0,0 +1,19 @@
+// server/repository — Database access layer (CRUD)
+//
+// Each sub-module owns the queries for one domain entity.
+// All functions accept `&AnyPool` and return `Result<_, DbError>`,
+// so the caller never has to think about raw SQL or connection management.
+//
+// Design choices:
+//   - Free functions (not struct methods) keep the API simple.
+//   - `sqlx::query()` is used instead of `sqlx::query!()` because the
+//     compile-time macro requires a live database at build time.
+//   - All integer columns are read as `i64` (the AnyPool normalizes
+//     INTEGER/BIGINT to 64-bit integers internally).
+//   - Port numbers are stored as i64 in the DB and cast to u16 in Rust
+//     (safe because valid port numbers 0–65535 fit in u16).
+
+pub mod applications;
+pub mod hosts;
+pub mod networks;
+pub mod ports;