feat: add /v1/completions legacy endpoint + show all 3 OpenAI endpoints in dashboard

- New route /v1/completions: accepts prompt string (legacy) + messages array
  Normalizes prompt format to chat/completions format automatically
- EndpointPageClient: Added 3rd card (Completions Legacy) in Core APIs section
  Dashboard now shows: /v1/chat/completions, /v1/responses, /v1/completions
- i18n: completionsLegacy/completionsLegacyDesc synced to 30 languages

src/app/api/v1/completions/route.ts

@@ -0,0 +1,97 @@
+import { CORS_ORIGIN, CORS_HEADERS } from "@/shared/utils/cors";
+import { handleChat } from "@/sse/handlers/chat";
+import { initTranslators } from "@omniroute/open-sse/translator/index.ts";
+import { createInjectionGuard } from "@/middleware/promptInjectionGuard";
+
+let initPromise = null;
+const injectionGuard = createInjectionGuard();
+
+function ensureInitialized() {
+  if (!initPromise) {
+    initPromise = Promise.resolve(initTranslators()).then(() => {
+      console.log("[SSE] Translators initialized");
+    });
+  }
+  return initPromise;
+}
+
+/**
+ * Handle CORS preflight
+ */
+export async function OPTIONS() {
+  return new Response(null, {
+    headers: {
+      "Access-Control-Allow-Origin": CORS_ORIGIN,
+      "Access-Control-Allow-Methods": "GET, POST, OPTIONS",
+      "Access-Control-Allow-Headers": "*",
+    },
+  });
+}
+
+/**
+ * POST /v1/completions — Legacy OpenAI Completions API
+ *
+ * Accepts both the modern chat format (messages[]) and the legacy
+ * text-completions format (prompt string). Legacy requests are
+ * automatically normalized to chat/completions format before routing.
+ *
+ * @see https://platform.openai.com/docs/api-reference/completions
+ */
+export async function POST(request: Request) {
+  await ensureInitialized();
+
+  // Prompt injection guard
+  try {
+    const cloned = request.clone();
+    const body = await cloned.json().catch(() => null);
+    if (body) {
+      const { blocked, result } = injectionGuard(body);
+      if (blocked) {
+        return new Response(
+          JSON.stringify({
+            error: {
+              message: "Request blocked: potential prompt injection detected",
+              type: "injection_detected",
+              code: "SECURITY_001",
+              detections: result.detections.length,
+            },
+          }),
+          { status: 400, headers: { ...CORS_HEADERS, "Content-Type": "application/json" } }
+        );
+      }
+
+      // Normalize legacy completions format: { prompt, model } → { messages, model }
+      // If the body has `prompt` but no `messages`, convert to chat format.
+      if (body.prompt !== undefined && !body.messages) {
+        const prompt = Array.isArray(body.prompt) ? body.prompt.join("\n") : String(body.prompt);
+        const normalized = {
+          ...body,
+          messages: [{ role: "user", content: prompt }],
+        };
+        delete normalized.prompt;
+
+        const newRequest = new Request(request.url, {
+          method: request.method,
+          headers: request.headers,
+          body: JSON.stringify(normalized),
+        });
+        return await handleChat(newRequest);
+      }
+    }
+  } catch (error) {
+    console.error("[SECURITY] Prompt injection guard failed:", error);
+    return new Response(
+      JSON.stringify({
+        error: {
+          message: "Security validation temporarily unavailable",
+          type: "security_guard_unavailable",
+          code: "SECURITY_002",
+        },
+      }),
+      { status: 503, headers: { ...CORS_HEADERS, "Content-Type": "application/json" } }
+    );
+  }
+
+  // Standard path: body already has messages[] (chat format)
+  return await handleChat(request);
+}