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/server/services/ai-assignment.ts

@@ -5,7 +5,7 @@
  * to generate optimal assignment suggestions.
  */
 
-import { getOpenAI, AI_MODELS } from '@/lib/openai'
+import { getOpenAI, getConfiguredModel } from '@/lib/openai'
 import {
   anonymizeForAI,
   deanonymizeResults,
@@ -153,8 +153,10 @@ async function callAIForAssignments(
   // Build the user prompt
   const userPrompt = buildAssignmentPrompt(anonymizedData, constraints)
 
+  const model = await getConfiguredModel()
+
   const response = await openai.chat.completions.create({
-    model: AI_MODELS.ASSIGNMENT,
+    model,
     messages: [
       { role: 'system', content: ASSIGNMENT_SYSTEM_PROMPT },
       { role: 'user', content: userPrompt },