Implement Phase 1 backend: FastAPI upload API with MinIO and PostgreSQL

- Add FastAPI app with CORS and auth middleware (Bearer token)
- Add POST /api/v1/transactions/upload endpoint (multipart/form-data)
- Add MinIO storage service with date-partitioned object keys
- Add PostgreSQL database service for transaction metadata
- Add Dockerfile and docker-compose.yml for deployment
- Add config.example.yaml template

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

config.example.yaml

@@ -0,0 +1,18 @@
+# SnapLedger Backend Configuration
+# Copy this file to config.yaml and fill in your credentials.
+
+minio:
+  endpoint: "https://minio.example.com:9000"
+  bucket: "snapledger"
+  access_key: "YOUR_MINIO_ACCESS_KEY"
+  secret_key: "YOUR_MINIO_SECRET_KEY"
+
+postgresql:
+  host: "localhost"
+  port: 5432
+  database: "snapledger"
+  user: "snapledger_user"
+  password: "YOUR_POSTGRES_PASSWORD"
+
+api:
+  token: "YOUR_API_TOKEN"