fix: auto-detect data directory in monorepo without DATA_DIR env var (#117)

- getDataDir() now detects monorepo by checking for ../docker-compose.yml
- DATA_DIR env var removed from .env and .env.example (no longer needed for local dev)
- Docker compose files explicitly set DATA_DIR=/app/data for containers
- Updated tests for monorepo detection logic

.env.example

@@ -13,11 +13,6 @@ PORT=3000
 CORS_ORIGINS=http://localhost:4174
 LOG_LEVEL=info
 
-# Data directory (where database, images, and state files are stored)
-# Default: ./data relative to process.cwd()
-# For local dev (cd backend && npm run dev), set to ../data so dev and prod share the same folder
-# DATA_DIR=../data
-
 # Timezone for scheduled reminders (e.g., Europe/Berlin, America/New_York)
 TZ=Europe/Berlin