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

backend/src/utils/server-config.ts

@@ -6,6 +6,7 @@
 import { existsSync, mkdirSync } from "node:fs";
 import { resolve } from "node:path";
 import type { CookieSerializeOptions } from "@fastify/cookie";
+import { getDataDir } from "../db/db-utils.js";
 
 /**
  * Parse comma-separated CORS origins string
@@ -81,8 +82,7 @@ export function buildAppConfig(options: AppConfigOptions): AppConfig {
  * Ensure images directory exists
  */
 export function ensureImagesDirectory(cwd?: string): string {
-	const basePath = cwd || process.cwd();
-	const imagesDir = resolve(basePath, "data/images");
+	const imagesDir = resolve(getDataDir(cwd), "images");
 	if (!existsSync(imagesDir)) {
 		mkdirSync(imagesDir, { recursive: true });
 	}