feat(ai-client): multi-provider compat — native Anthropic/Google + URL tolerance

- TEXT/VISION: add native Anthropic & Google Gemini paths via Vercel AI SDK,
  selectable through TEXT_PROVIDER / VISION_PROVIDER (default openai_compatible)
- IMAGE: expand to openai (gpt-image) / google (Nano Banana) via AI SDK
  alongside the existing Runware task-array and OpenAI-compatible REST paths
- normalizeBaseUrl: tolerate URLs with/without /v1 (or /chat/completions);
  append the per-protocol version segment only for bare hosts
- config: readProvider() reads *_PROVIDER; types: ProviderProtocol + provider?
- deps: @ai-sdk/anthropic, @ai-sdk/google; docs in .env.example + README

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/config.ts

@@ -1,4 +1,16 @@
-import type { EngineConfig, TtsConfig } from "@infiplot/types";
+import type {
+  EngineConfig,
+  ProviderProtocol,
+  TtsConfig,
+} from "@infiplot/types";
+
+const VALID_PROTOCOLS = [
+  "openai_compatible",
+  "anthropic",
+  "google",
+  "openai",
+  "runware",
+] as const;
 
 function readVar(name: string): string {
   const v = process.env[name];
@@ -11,6 +23,21 @@ function readOptionalVar(name: string): string | undefined {
   return v && v.length > 0 ? v : undefined;
 }
 
+// Optional *_PROVIDER selector. Unset → undefined, and each ai-client adapter
+// applies its own default (text/vision → openai_compatible; image → inferred
+// from the base URL). Validated eagerly so a typo fails fast at boot rather
+// than mid-request.
+function readProvider(name: string): ProviderProtocol | undefined {
+  const v = readOptionalVar(name)?.trim().toLowerCase();
+  if (!v) return undefined;
+  if ((VALID_PROTOCOLS as readonly string[]).includes(v)) {
+    return v as ProviderProtocol;
+  }
+  throw new Error(
+    `Invalid ${name}: "${v}". Must be one of: ${VALID_PROTOCOLS.join(", ")}`,
+  );
+}
+
 function loadTtsConfig(): TtsConfig | undefined {
   const baseUrl = readOptionalVar("TTS_BASE_URL");
   const apiKey = readOptionalVar("TTS_API_KEY");
@@ -28,16 +55,19 @@ export function loadEngineConfig(headers?: Headers): EngineConfig {
       baseUrl: readVar("TEXT_BASE_URL"),
       apiKey: readVar("TEXT_API_KEY"),
       model: readVar("TEXT_MODEL"),
+      provider: readProvider("TEXT_PROVIDER"),
     },
     image: {
       baseUrl: readVar("IMAGE_BASE_URL"),
       apiKey: readVar("IMAGE_API_KEY"),
       model: readVar("IMAGE_MODEL"),
+      provider: readProvider("IMAGE_PROVIDER"),
     },
     vision: {
       baseUrl: readVar("VISION_BASE_URL"),
       apiKey: readVar("VISION_API_KEY"),
       model: readVar("VISION_MODEL"),
+      provider: readProvider("VISION_PROVIDER"),
     },
     tts: loadTtsConfig(),
     mockImage: readOptionalVar("MOCK_IMAGE") === "true",