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/medications.ts

@@ -5,13 +5,14 @@ import { and, eq, like } from "drizzle-orm";
 import type { FastifyInstance, FastifyReply, FastifyRequest } from "fastify";
 import { z } from "zod";
 import { db } from "../db/client.js";
+import { getDataDir } from "../db/db-utils.js";
 import { doseTracking, medications } from "../db/schema.js";
 import { getAnonymousUserId, requireAuth } from "../plugins/auth.js";
 import { env } from "../plugins/env.js";
 import type { AuthUser } from "../types/fastify.js";
 import { type Intake, parseIntakesJson, parseLocalDateTime, parseTakenByJson } from "../utils/scheduler-utils.js";
 
-const IMAGES_DIR = resolve(process.cwd(), "data/images");
+const IMAGES_DIR = resolve(getDataDir(), "images");
 
 // New intake schema with per-intake takenBy
 const intakeSchema = z.object({