scaffold: Backend API routes, DB queries, and middleware stubs

88 handler stubs across 13 route files, 66 DB query stubs across
11 modules, auth/license extractors, and rate limit middleware.
All bodies are todo!() — ready for Phase 1b implementation.

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

backend/src/db/users.rs

@@ -0,0 +1,30 @@
+use sqlx::PgPool;
+use uuid::Uuid;
+use anyhow::Result;
+
+// TODO: Define User struct (id, email, password_hash, display_name, avatar_url, created_at, updated_at, last_login_at)
+
+/// Create a new user record.
+pub async fn create_user(pool: &PgPool, email: &str, password_hash: &str, display_name: &str) -> Result<Uuid> {
+    todo!()
+}
+
+/// Fetch a user by their primary key.
+pub async fn get_user_by_id(pool: &PgPool, user_id: Uuid) -> Result<()> {
+    todo!()
+}
+
+/// Fetch a user by email address (for login lookups).
+pub async fn get_user_by_email(pool: &PgPool, email: &str) -> Result<()> {
+    todo!()
+}
+
+/// Update mutable user profile fields.
+pub async fn update_user(pool: &PgPool, user_id: Uuid, display_name: Option<&str>, avatar_url: Option<&str>) -> Result<()> {
+    todo!()
+}
+
+/// Bump the last_login_at timestamp for a user.
+pub async fn update_last_login(pool: &PgPool, user_id: Uuid) -> Result<()> {
+    todo!()
+}