fix: unify data directory for dev and prod environments (#116)

Add DATA_DIR env var support to configure the data directory path.
All hardcoded resolve(cwd, 'data') paths now use a central getDataDir()
function from db-utils.ts that checks DATA_DIR first, falling back to
resolve(cwd, 'data').

This prevents local dev (cd backend && npm run dev) from creating a
separate backend/data/ directory instead of using the root data/ folder.

Changes:
- Add getDataDir() to db-utils.ts as single source of truth
- Update all 8 source files that reference the data directory
- Add dotenv fallback to ../.env for local dev from backend/
- Add DATA_DIR documentation to .env.example
- Add 7 new tests for getDataDir and getDbPaths with DATA_DIR
- 493 tests pass, TypeScript clean

.env.example

@@ -13,6 +13,11 @@ 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