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/routes/auth.ts

@@ -4,6 +4,7 @@ import { eq } from "drizzle-orm";
 import type { FastifyInstance } from "fastify";
 import { z } from "zod";
 import { db } from "../db/client.js";
+import { getDataDir } from "../db/db-utils.js";
 import { refreshTokens, users } from "../db/schema.js";
 import { getAuthState, requireAuth } from "../plugins/auth.js";
 import type { AuthUser } from "../types/fastify.js";
@@ -476,7 +477,7 @@ export async function authRoutes(app: FastifyInstance) {
 			// Save file
 			const fs = await import("node:fs/promises");
 			const path = await import("node:path");
-			const imagesDir = path.join(process.cwd(), "data", "images");
+			const imagesDir = path.join(getDataDir(), "images");
 			await fs.mkdir(imagesDir, { recursive: true });
 
 			const buffer = await data.toBuffer();
@@ -523,7 +524,7 @@ export async function authRoutes(app: FastifyInstance) {
 			const fs = await import("node:fs/promises");
 			const path = await import("node:path");
 			try {
-				await fs.unlink(path.join(process.cwd(), "data", "images", user.avatarUrl));
+				await fs.unlink(path.join(getDataDir(), "images", user.avatarUrl));
 			} catch {
 				// Ignore if file doesn't exist
 			}
@@ -556,7 +557,7 @@ export async function authRoutes(app: FastifyInstance) {
 				const fs = await import("node:fs/promises");
 				const path = await import("node:path");
 				try {
-					await fs.unlink(path.join(process.cwd(), "data", "images", user.avatarUrl));
+					await fs.unlink(path.join(getDataDir(), "images", user.avatarUrl));
 				} catch {
 					// Ignore if file doesn't exist
 				}