Initial scaffold: Flask/SQLite backend, React/Vite PWA frontend

Backend: receipt upload -> LLM vision extraction (OpenAI-compatible,
provider-agnostic), item review/edit, per-group splitting against
Cospend projects/members, highlight+upload via WebDAV, public share
link, bill creation via Cospend's OCS API (verified against real
source, not just doc summaries).

Frontend: capture -> review -> group -> summary flow as an installable
PWA.

install.sh / run.sh (venv + npm, tmux session) instead of Docker, per
the ~/Projects/gain pattern.

backend/app/__init__.py

@@ -0,0 +1,27 @@
+import os
+
+from flask import Flask
+from flask_cors import CORS
+
+from .config import Config
+from .db import init_db
+
+
+def create_app() -> Flask:
+    app = Flask(__name__, instance_relative_config=True)
+    app.config.from_object(Config)
+
+    os.makedirs(app.instance_path, exist_ok=True)
+    os.makedirs(Config.UPLOAD_DIR, exist_ok=True)
+
+    # Frontend runs on a different origin (vite dev server) during
+    # development; lock this down to that origin in production.
+    CORS(app, resources={r"/api/*": {"origins": Config.CORS_ORIGIN}})
+
+    init_db(Config.DATABASE_PATH)
+
+    from .routes import bp as api_bp
+
+    app.register_blueprint(api_bp, url_prefix="/api")
+
+    return app