AI Vision: multi-provider config, model testing, per-client AI preferences

Backend:
- Add ai_provider + ai_model columns to APIClient model
- New admin_ai router: GET /admin/api/ai/status + POST /admin/api/ai/test
- Update ai_vision.py: provider_override + model_override support
- Update pipeline: load client AI prefs for image processing
- ClientUpdate/ClientResponse schemas include ai_provider/ai_model

Frontend:
- New AIConfigSection component: provider list, model dropdowns,
  test button with live results (no page refresh needed)
- AI types (ai.types.ts) + API (api/ai.ts)
- APIClient types extended with ai_provider/ai_model

app/models/client.py

@@ -47,6 +47,10 @@ class APIClient(Base):
     quota_storage_mb = Column(Integer, default=500, nullable=False)
     quota_images = Column(Integer, default=1000, nullable=False)
 
+    # ── AI Preferences (per-client provider/model override) ────
+    ai_provider = Column(String(64), nullable=True)
+    ai_model = Column(String(256), nullable=True)
+
     # ── Timestamps ────────────────────────────────────────────
     created_at = Column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc))
     updated_at = Column(