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/routers/admin/__init__.py

@@ -7,6 +7,7 @@ from devplacepy.routers.admin import (
     backups,
     bots,
     containers,
+    gateway_configs,
     issues,
     media,
     news,
@@ -30,5 +31,6 @@ router.include_router(issues.router)
 router.include_router(auditlog.router)
 router.include_router(backups.router)
 router.include_router(bots.router)
+router.include_router(gateway_configs.router)
 router.include_router(services.router, prefix="/services")
 router.include_router(containers.router, prefix="/containers")