feat: add provider fallback chain with priority system

src/lib/llm/providers/index.ts

@@ -0,0 +1,63 @@
+import type { ComparisonRequest, ComparisonResult } from "../types";
+import type { SearchResult } from "./tavily";
+import { generateComparisonWithResearch, generateComparison } from "./openai";
+import { synthesizeResearch } from "./perplexity";
+
+export interface Provider {
+  name: string;
+  hasSearch: boolean;
+  synthesize: (
+    request: ComparisonRequest,
+    searchResults: Record<string, SearchResult[]>
+  ) => Promise<ComparisonResult>;
+}
+
+export function getActiveProvider(): Provider {
+  const hasTavily = !!process.env.TAVILY_API_KEY;
+  const hasPerplexity = !!process.env.PERPLEXITY_API_KEY;
+  const hasOpenAI = !!process.env.OPENAI_API_KEY;
+
+  if (hasTavily && hasPerplexity) {
+    console.log("[llm] Using provider: Tavily search + Perplexity synthesis");
+    return {
+      name: "Tavily + Perplexity",
+      hasSearch: true,
+      synthesize: synthesizeResearch,
+    };
+  }
+
+  if (hasTavily && hasOpenAI) {
+    console.log("[llm] Using provider: Tavily search + OpenAI synthesis");
+    return {
+      name: "Tavily + OpenAI",
+      hasSearch: true,
+      synthesize: generateComparisonWithResearch,
+    };
+  }
+
+  if (hasOpenAI) {
+    console.log("[llm] Using provider: OpenAI only (no web search)");
+    return {
+      name: "OpenAI",
+      hasSearch: false,
+      synthesize: async (request) => generateComparison(request),
+    };
+  }
+
+  console.warn(
+    "[llm] No API keys configured. Research will fail at synthesis."
+  );
+  return {
+    name: "None",
+    hasSearch: false,
+    synthesize: async () => {
+      throw new Error(
+        "No LLM provider configured. Set OPENAI_API_KEY, TAVILY_API_KEY, or PERPLEXITY_API_KEY."
+      );
+    },
+  };
+}
+
+export { searchItem, type SearchResult } from "./tavily";
+export { generateComparison, generateComparisonWithResearch } from "./openai";
+export { synthesizeResearch } from "./perplexity";