feat: add provider and model routing tables, admin UI, and audit category for gateway

Implement the multi-provider routing system for the OpenAI gateway, including two new database tables (`gateway_providers`, `gateway_models`) ensured at init, a new admin page at `/admin/gateway` with full CRUD for providers and model routes, and a `"gateway"` audit category mapped to `"ai"`. The routing layer sits transparently on top of the existing single-provider default path: unmatched model names fall through unchanged, while matched routes forward to the configured provider with their own pricing economy, vision model, and context window. Cross-worker cache invalidation uses a shared `_ROUTING_CACHE` bumped via `"gateway_routing"` cache version.

tests/unit/services/openai_gateway/gateway.py

@@ -112,6 +112,47 @@ def test_model_is_forced(local_db, monkeypatch):
     assert rt._client.calls[-1][1]["model"] == "deepseek-chat"
 
 
+def test_model_route_overrides_upstream(local_db, monkeypatch):
+    from devplacepy.services.openai_gateway import routing
+
+    monkeypatch.setattr(gwmod.httpx, "AsyncClient", FakeClient_openai_gateway)
+    routing.provider_store.set(
+        routing.ProviderIn(
+            name="gwroute",
+            base_url="https://routed.example/v1/chat/completions",
+            api_key="routed-key",
+        )
+    )
+    routing.model_store.set(
+        routing.ModelRouteIn(
+            source_model="custom-pro",
+            provider="gwroute",
+            target_model="vendor/pro",
+            kind="chat",
+            price_output_per_m=9.0,
+        )
+    )
+    try:
+        svc = GatewayService()
+        cfg = svc.effective_config()
+        cfg["gateway_vision_enabled"] = False
+        rt = svc.runtime()
+        run_async(
+            rt.handle_chat(
+                {"model": "custom-pro", "messages": [{"role": "user", "content": "hi"}]},
+                cfg,
+                ("guest", "test"),
+                "test",
+            )
+        )
+        url, body = rt._client.calls[-1]
+        assert str(url) == "https://routed.example/v1/chat/completions"
+        assert body["model"] == "vendor/pro"
+    finally:
+        routing.model_store.remove("custom-pro")
+        routing.provider_store.remove("gwroute")
+
+
 def test_vision_rewrites_image_to_text(local_db, monkeypatch):
     monkeypatch.setattr(gwmod.httpx, "AsyncClient", FakeClient_openai_gateway)
     svc = GatewayService()