feat: endpoint-aware model management + fix 3 bugs (#212, #213, #200)

Bug Fixes:
- #212: Auto-generate API_KEY_SECRET at startup (like JWT_SECRET)
- #213: Circuit breaker now scoped per-model instead of per-provider
- #200: Connectivity fallback for custom providers (Ollama, LM Studio)

Features:
- #204: API Format selector (Chat Completions / Responses API) for custom models
- #205: Combo endpoint field (chat / embeddings / images) in schema
- #206: Supported Endpoints checkboxes (chat, embeddings, images, audio)
- Custom models with endpoint tags appear in /v1/embeddings and /v1/images/generations
- Model catalog includes api_format, type, and supported_endpoints metadata
- Provider detail page shows badges for non-default endpoint configurations

Files changed: instrumentation.ts, combo.ts, validation.ts, models.ts,
schemas.ts, provider-models/route.ts, providers/[id]/page.tsx,
catalog.ts, embeddings/route.ts, images/generations/route.ts

src/shared/schemas/validation.ts

@@ -37,6 +37,7 @@ export const comboNodeSchema = z.object({
 export const comboSchema = z.object({
   name: z.string().min(1, "Combo name is required").max(100),
   model: z.string().min(1, "Model pattern is required"),
+  endpoint: z.enum(["chat", "embeddings", "images"]).default("chat"),
   strategy: z
     .enum(["priority", "weighted", "round-robin", "random", "least-used", "cost-optimized"])
     .default("priority"),

src/shared/validation/schemas.ts

@@ -300,6 +300,8 @@ export const providerModelMutationSchema = z.object({
   modelId: z.string().trim().min(1, "modelId is required").max(240),
   modelName: z.string().trim().max(240).optional(),
   source: z.string().trim().max(80).optional(),
+  apiFormat: z.enum(["chat-completions", "responses"]).default("chat-completions"),
+  supportedEndpoints: z.array(z.enum(["chat", "embeddings", "images", "audio"])).default(["chat"]),
 });
 
 const pricingFieldsSchema = z