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/plugins/env.ts

@@ -1,7 +1,10 @@
+import { existsSync } from "node:fs";
 import dotenv from "dotenv";
 import { z } from "zod";
 
-dotenv.config({ path: process.env.DOTENV_PATH || ".env" });
+// Load .env: try cwd first, then parent dir (for local dev running from backend/)
+const envPath = process.env.DOTENV_PATH || (existsSync(".env") ? ".env" : "../.env");
+dotenv.config({ path: envPath });
 
 const EnvSchema = z.object({
 	NODE_ENV: z.enum(["development", "production", "test"]).default("production"),