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.

devplacepy/services/openai_gateway/gateway.py

@@ -13,6 +13,7 @@ from fastapi.responses import JSONResponse, Response, StreamingResponse
 from devplacepy import stealth
 from devplacepy.services.openai_gateway import config
 from devplacepy.services.openai_gateway.reliability import CircuitBreaker, retry_send
+from devplacepy.services.openai_gateway.routing import chat_overlay, embed_overlay
 from devplacepy.services.openai_gateway.system_message import apply_system_directives
 from devplacepy.services.openai_gateway.usage import (
     GatewayUsageLedger,
@@ -213,6 +214,10 @@ class GatewayRuntime:
         self, body: dict, cfg: dict, owner: tuple, user_agent: str, log=None
     ):
         log = log or (lambda message: None)
+        overlay = chat_overlay(body.get("model"), cfg)
+        if overlay:
+            cfg = {**cfg, **overlay}
+            log(f"routed model {body.get('model')!r} -> {cfg['gateway_model']!r}")
         client, sem = self._ensure(cfg)
         pricing = pricing_from_cfg(cfg)
         context_map = parse_context_map(cfg.get("gateway_model_context_map"))
@@ -365,6 +370,10 @@ class GatewayRuntime:
         self, body: dict, cfg: dict, owner: tuple, user_agent: str, log=None
     ):
         log = log or (lambda message: None)
+        overlay = embed_overlay(body.get("model"), cfg)
+        if overlay:
+            cfg = {**cfg, **overlay}
+            log(f"routed embed model {body.get('model')!r} -> {cfg['gateway_embed_model']!r}")
         if not cfg["gateway_embed_enabled"]:
             from devplacepy.services.audit import record as audit
             from devplacepy.services.openai_gateway.usage import audit_actor_for