refactor: remove GATEWAY_USER_TOKENS, fix review feedback

GATEWAY_USER_TOKENS never went to production — replaced entirely by
DB-backed user management via /api/admin/users and /api/tokens.

Removed:
- UserTokenConfig struct and GATEWAY_USER_TOKENS env var parsing
- user_tokens field from GatewayConfig
- GatewayChannel::new_multi_auth() constructor
- Env-var user migration block in main.rs (~90 lines)
- multi_tenant auto-detection from GATEWAY_USER_TOKENS (now runtime
  via db.has_any_users() in app.rs)

Review fixes (zmanian):
- User ID generation: UUID instead of display-name derivation (#1)
- Invitation accept moved to public router (no auth needed) (#3)
- libSQL get_invitation_by_hash aligned with postgres: filters
  status='pending' AND expires_at > now (#4)
- UUID parse: returns DatabaseError::Serialization instead of
  unwrap_or_default (#7)
- PostgreSQL SELECT * replaced with explicit column lists (#8)
- Sort order aligned (both backends use DESC) (#6)

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

src/channels/web/mod.rs

@@ -124,49 +124,6 @@ impl GatewayChannel {
         }
     }
 
-    /// Create a gateway channel with a pre-built multi-user auth state.
-    pub fn new_multi_auth(config: GatewayConfig, auth: MultiAuthState) -> Self {
-        let auth = CombinedAuthState {
-            env_auth: auth,
-            db_auth: None,
-        };
-        let state = Arc::new(GatewayState {
-            msg_tx: tokio::sync::RwLock::new(None),
-            sse: Arc::new(SseManager::new()),
-            workspace: None,
-            workspace_pool: None,
-            session_manager: None,
-            log_broadcaster: None,
-            log_level_handle: None,
-            extension_manager: None,
-            tool_registry: None,
-            store: None,
-            job_manager: None,
-            prompt_queue: None,
-            scheduler: None,
-            default_user_id: config.user_id.clone(),
-            shutdown_tx: tokio::sync::RwLock::new(None),
-            ws_tracker: Some(Arc::new(ws::WsConnectionTracker::new())),
-            llm_provider: None,
-            skill_registry: None,
-            skill_catalog: None,
-            chat_rate_limiter: server::PerUserRateLimiter::new(30, 60),
-            oauth_rate_limiter: server::RateLimiter::new(10, 60),
-            registry_entries: Vec::new(),
-            cost_guard: None,
-            routine_engine: Arc::new(tokio::sync::RwLock::new(None)),
-            startup_time: std::time::Instant::now(),
-            webhook_rate_limiter: server::RateLimiter::new(10, 60),
-            active_config: server::ActiveConfigSnapshot::default(),
-        });
-
-        Self {
-            config,
-            state,
-            auth,
-        }
-    }
-
     /// Helper to rebuild state, copying existing fields and applying a mutation.
     fn rebuild_state(&mut self, mutate: impl FnOnce(&mut GatewayState)) {
         let mut new_state = GatewayState {