feat: auto-build FAISS index on startup if missing

Add lifespan handler that checks for FAISS index at startup
and automatically builds it if not found. This ensures the
service works on fresh deployments without manual indexing.

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

app/main.py

@@ -1,5 +1,6 @@
 import logging
 import uuid
+from contextlib import asynccontextmanager
 
 from fastapi import FastAPI, HTTPException, BackgroundTasks
 from fastapi.middleware.cors import CORSMiddleware
@@ -7,7 +8,7 @@ from fastapi.responses import StreamingResponse
 
 from app.auth import ServiceAuthDependency
 from app.config import settings, MAX_MESSAGE_LENGTH
-from app.embeddings import RetrieverDependency, IndexerDependency, get_retriever, reset_retriever
+from app.embeddings import RetrieverDependency, IndexerDependency, get_retriever, reset_retriever, get_indexer
 from app.embeddings.retriever import RetrievedChunk
 from app.intent import IntentClassifierDependency
 from app.llm import AdapterDependency, LLMError, llm_exception_to_http
@@ -31,11 +32,35 @@ logging.basicConfig(
 )
 logger = logging.getLogger(__name__)
 
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    """Startup and shutdown lifecycle management."""
+    # Startup: auto-index if FAISS index is missing
+    retriever = get_retriever()
+    if not retriever.is_loaded:
+        logger.info("FAISS index not found, building index on startup...")
+        try:
+            indexer = get_indexer()
+            result = await indexer.build_index()
+            reset_retriever()  # Reset to load newly built index
+            logger.info(f"Startup indexing completed: {result.chunks_indexed} chunks from {result.artifacts_processed} artifacts")
+        except Exception as e:
+            logger.error(f"Startup indexing failed: {e}")
+    else:
+        logger.info(f"FAISS index loaded: {retriever.index_size} vectors")
+
+    yield  # App runs here
+
+    # Shutdown: nothing to clean up
+
+
 # Create FastAPI app
 app = FastAPI(
     title="Tyndale AI Service",
     description="LLM Chat Service for algorithmic trading support",
     version="0.1.0",
+    lifespan=lifespan,
 )
 
 # Add CORS middleware