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/app/gateway/routers/agents.py

@@ -14,7 +14,7 @@ from deerflow.config.paths import get_paths
 from deerflow.runtime.user_context import get_effective_user_id
 
 logger = logging.getLogger(__name__)
-router = APIRouter(prefix="/api", tags=["agents"])
+router = APIRouter(tags=["agents"])
 
 AGENT_NAME_PATTERN = re.compile(r"^[A-Za-z0-9-]+$")