Extend team enforcement to Atlas and Archive routes, update schema reference

- Atlas: add requireTeam() at router level; replace client ?teams= param
  parsing with req.teamScope in /metrics, /status, and /sync endpoints
- Archive: add requireTeam() at router level; replace client ?teams= param
  parsing with req.teamScope in GET / and GET /stats endpoints
- db-schema.sql: add impersonate_user_id column to sessions table reference

The frontend still sends ?teams= as a query param to these endpoints
(harmless no-op since backend ignores it). Frontend cleanup deferred
to avoid churn in the 7000-line ReportingPage component.

backend/db-schema.sql

@@ -87,7 +87,8 @@ CREATE TABLE IF NOT EXISTS sessions (
     session_id VARCHAR(255) UNIQUE NOT NULL,
     user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
     expires_at TIMESTAMPTZ NOT NULL,
-    created_at TIMESTAMPTZ DEFAULT NOW()
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    impersonate_user_id INTEGER REFERENCES users(id) ON DELETE SET NULL
 );
 
 CREATE INDEX IF NOT EXISTS idx_sessions_session_id ON sessions(session_id);