feat(core): add core layer with config, clients, exceptions, models, and app factory

src/main.py

@@ -0,0 +1,43 @@
+"""FastAPI app factory. Composes all domain routers and middleware."""
+
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+from src.core.exceptions import (
+    ChunkingError,
+    BenchmarkError,
+    chunking_exception_handler,
+    benchmark_exception_handler,
+)
+
+
+def create_app() -> FastAPI:
+    """Create and configure the FastAPI application.
+
+    Mounts domain routers and registers exception handlers.
+    Routers are imported lazily — domains are added in later phases.
+    """
+    app = FastAPI(
+        title="RAG Chunking Benchmarker",
+        description="Benchmark five chunking strategies on regulatory documents. "
+        "Compare retrieval quality, answer faithfulness, and cost.",
+        version="0.1.0",
+    )
+
+    # CORS — allow all origins for development
+    app.add_middleware(
+        CORSMiddleware,
+        allow_origins=["*"],
+        allow_credentials=True,
+        allow_methods=["*"],
+        allow_headers=["*"],
+    )
+
+    # Register exception handlers
+    app.add_exception_handler(ChunkingError, chunking_exception_handler)
+    app.add_exception_handler(BenchmarkError, benchmark_exception_handler)
+
+    return app
+
+
+app = create_app()