fix(_deps): fail-fast on missing services.sam21.base_url

Align sam21() with nuwa()/marker()/yolo(): require base_url to be present
in config rather than silently defaulting. Misconfiguration now raises
KeyError at factory call instead of masking as a transport error.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/presa_mano/api/_deps.py

@@ -32,7 +32,7 @@ def yolo() -> YoloClient:
 def sam21() -> Sam21Client:
     c = _svc(get_config(), "sam21")
     return Sam21Client(
-        base_url=c.get("base_url", "http://localhost:8002"),
+        base_url=c["base_url"],
         timeout_s=float(c.get("timeout_s", 10.0)),
     )