Use admin-configured AI model and add GPT-5/o-series options

- Add getConfiguredModel() that reads ai_model from SystemSettings
- AI assignment and mentor matching now use the admin-selected model
- Remove duplicate OpenAI client in mentor-matching (use shared singleton)
- Add GPT-5, GPT-5 Mini, o3, o3 Mini, o4 Mini to model dropdown

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/lib/openai.ts

@@ -107,3 +107,18 @@ export const AI_MODELS = {
   ASSIGNMENT: 'gpt-4o', // Best for complex reasoning
   QUICK: 'gpt-4o-mini', // Faster, cheaper for simple tasks
 } as const
+
+/**
+ * Get the admin-configured AI model from SystemSettings.
+ * Falls back to the provided default if not configured.
+ */
+export async function getConfiguredModel(fallback: string = AI_MODELS.ASSIGNMENT): Promise<string> {
+  try {
+    const setting = await prisma.systemSettings.findUnique({
+      where: { key: 'ai_model' },
+    })
+    return setting?.value || process.env.OPENAI_MODEL || fallback
+  } catch {
+    return process.env.OPENAI_MODEL || fallback
+  }
+}