feat(gateway): dual-mount legacy routers on /api and /api/v1 (Stage 1 PR5)

Strip /api prefix from 13 legacy router APIRouter() declarations and dual-mount
each on prefix="/api" (backward compat) and prefix="/api/v1" (versioned surface)
in app.py. Auth, service-accounts, api-keys, assistants-compat remain single-mount.
Update 10 test files to pass prefix="/api" when directly including stripped routers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/test_custom_agent.py

@@ -382,7 +382,7 @@ def _make_test_app(tmp_path: Path):
     from app.gateway.routers.agents import router
 
     app = FastAPI()
-    app.include_router(router)
+    app.include_router(router, prefix="/api")
     return app