feat(api): add Leptos server functions bridging client and server

- Add src/api/ module with server functions for networks, hosts, applications
- Each server function retrieves the pool via use_context::<AnyPool>()
- Pool is injected via provide_context in two places in main.rs:
  * leptos_routes_with_context: for SSR renders and inline server fn calls
  * handle_server_fns_with_context on /api/*fn_name: for WASM client calls
- create_host validates IP against network CIDR before inserting
- create_network validates CIDR format before inserting

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

src/api/networks.rs

@@ -0,0 +1,67 @@
+// api/networks.rs — Server functions for networks
+
+use leptos::prelude::*;
+
+use crate::models::Network;
+
+// ─── Queries ──────────────────────────────────────────────────────────────────
+
+/// Returns all networks from the database.
+///
+/// Called by the Networks page to populate the list.
+#[server]
+pub async fn get_networks() -> Result<Vec<Network>, ServerFnError> {
+    use sqlx::AnyPool;
+    use crate::server::repository::networks as repo;
+
+    // `use_context` retrieves a value previously registered with `provide_context`.
+    // The pool was injected in main.rs before every request.
+    // `ok_or_else` converts `None` into an error (defensive: should never happen).
+    let pool = use_context::<AnyPool>()
+        .ok_or_else(|| ServerFnError::new("Database pool not found in context"))?;
+
+    // Propagate any DB error as a ServerFnError so the client sees a clean message.
+    repo::list_networks(&pool)
+        .await
+        .map_err(|e| ServerFnError::new(e.to_string()))
+}
+
+// ─── Mutations ────────────────────────────────────────────────────────────────
+
+/// Creates a new network with the given CIDR block.
+///
+/// Returns the created record (with its auto-generated id).
+/// Returns an error if the CIDR is malformed or already exists.
+#[server]
+pub async fn create_network(cidr: String) -> Result<Network, ServerFnError> {
+    use sqlx::AnyPool;
+    use crate::server::{repository::networks as repo, validation::validate_cidr};
+
+    let pool = use_context::<AnyPool>()
+        .ok_or_else(|| ServerFnError::new("Database pool not found in context"))?;
+
+    // Validate the CIDR before touching the database.
+    // Example of a valid CIDR: "192.168.1.0/24"
+    validate_cidr(&cidr).map_err(|e| ServerFnError::new(e.to_string()))?;
+
+    repo::create_network(&pool, &cidr)
+        .await
+        .map_err(|e| ServerFnError::new(e.to_string()))
+}
+
+/// Deletes a network by id.
+///
+/// Also deletes all hosts in that network (via `ON DELETE CASCADE`).
+/// Returns `true` if the network existed and was deleted.
+#[server]
+pub async fn delete_network(id: i64) -> Result<bool, ServerFnError> {
+    use sqlx::AnyPool;
+    use crate::server::repository::networks as repo;
+
+    let pool = use_context::<AnyPool>()
+        .ok_or_else(|| ServerFnError::new("Database pool not found in context"))?;
+
+    repo::delete_network(&pool, id)
+        .await
+        .map_err(|e| ServerFnError::new(e.to_string()))
+}