chore: switch to DeepSeek API + sentence-transformers for free deployment

- Replace OpenAI LLM with DeepSeek (deepseek-chat) via OpenAI-compatible client
- Replace OpenAI embeddings with local sentence-transformers (all-MiniLM-L6-v2)
- Pre-download embedding model during Docker build
- Add .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/handlers/user.py

@@ -2,13 +2,13 @@ from pathlib import Path
 from telegram import Update
 from telegram.ext import ContextTypes
 from openai import AsyncOpenAI
-from config import OPENAI_API_KEY, OPENAI_MODEL, ADMIN_TELEGRAM_ID
+from config import DEEPSEEK_API_KEY, DEEPSEEK_BASE_URL, DEEPSEEK_MODEL, ADMIN_TELEGRAM_ID
 import database as db
 import rag
 import document as doc_processor
 from guard import is_academic_topic, REJECT_MESSAGE
 
-_client = AsyncOpenAI(api_key=OPENAI_API_KEY)
+_client = AsyncOpenAI(api_key=DEEPSEEK_API_KEY, base_url=DEEPSEEK_BASE_URL)
 
 ACADEMIC_SYSTEM_PROMPT = (
     "Anda adalah asisten akademik personal untuk dosen di perguruan tinggi Indonesia. "
@@ -128,7 +128,7 @@ async def ask(update: Update, context: ContextTypes.DEFAULT_TYPE):
     await update.message.chat.send_action("typing")
 
     # Guard: hanya izinkan topik akademik
-    if not await is_academic_topic(question, _client, OPENAI_MODEL):
+    if not await is_academic_topic(question, _client, DEEPSEEK_MODEL):
         await update.message.reply_text(REJECT_MESSAGE)
         return
 
@@ -154,7 +154,7 @@ async def ask(update: Update, context: ContextTypes.DEFAULT_TYPE):
 
     try:
         response = await _client.chat.completions.create(
-            model=OPENAI_MODEL,
+            model=DEEPSEEK_MODEL,
             messages=messages,
             max_tokens=1500,
         )