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/index.ts

@@ -10,6 +10,7 @@ import sensible from "@fastify/sensible";
 import fastifyStatic from "@fastify/static";
 import Fastify, { type FastifyInstance } from "fastify";
 import { migrationsReady } from "./db/client.js";
+import { getDataDir } from "./db/db-utils.js";
 import { env } from "./plugins/env.js";
 import { authRoutes } from "./routes/auth.js";
 import { doseRoutes } from "./routes/doses.js";
@@ -66,7 +67,7 @@ export async function createApp(options?: {
 		accessTtlMinutes: options?.accessTtlMinutes ?? 15,
 		refreshTtlDays: options?.refreshTtlDays ?? 7,
 		isProduction: options?.isProduction ?? false,
-		imagesDir: options?.imagesDir ?? resolve(process.cwd(), "data/images"),
+		imagesDir: options?.imagesDir ?? resolve(getDataDir(), "images"),
 	};
 
 	const app = Fastify({