feat: admin secrets provisioning API + API documentation

- Add PUT/GET/DELETE /api/admin/users/{id}/secrets/{name} endpoints for
  application backends to provision per-user secrets (AES-256-GCM encrypted)
- Add secrets_store field to GatewayState with builder wiring
- Create docs/USER_MANAGEMENT_API.md with full API spec covering users,
  secrets, tokens, profile, and usage endpoints
- Update web gateway CLAUDE.md route table

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/channels/web/mod.rs

@@ -116,6 +116,7 @@ impl GatewayChannel {
             routine_engine: Arc::new(tokio::sync::RwLock::new(None)),
             startup_time: std::time::Instant::now(),
             active_config: server::ActiveConfigSnapshot::default(),
+            secrets_store: None,
         });
 
         Self {
@@ -169,6 +170,7 @@ impl GatewayChannel {
             routine_engine: Arc::clone(&self.state.routine_engine),
             startup_time: self.state.startup_time,
             active_config: self.state.active_config.clone(),
+            secrets_store: self.state.secrets_store.clone(),
         };
         mutate(&mut new_state);
         self.state = Arc::new(new_state);
@@ -292,6 +294,15 @@ impl GatewayChannel {
         self
     }
 
+    /// Inject the secrets store for admin secret provisioning.
+    pub fn with_secrets_store(
+        mut self,
+        store: Arc<dyn crate::secrets::SecretsStore + Send + Sync>,
+    ) -> Self {
+        self.rebuild_state(|s| s.secrets_store = Some(store));
+        self
+    }
+
     /// Inject the per-user workspace pool for multi-user mode.
     pub fn with_workspace_pool(mut self, pool: Arc<server::WorkspacePool>) -> Self {
         self.rebuild_state(|s| s.workspace_pool = Some(pool));