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/services/intake-reminder-scheduler.ts

@@ -3,6 +3,7 @@ import { resolve } from "node:path";
 import { and, eq, gte, lte } from "drizzle-orm";
 import nodemailer from "nodemailer";
 import { db } from "../db/client.js";
+import { getDataDir } from "../db/db-utils.js";
 import { doseTracking, medications } from "../db/schema.js";
 import { getDateLocale, getTranslations, type Language, t } from "../i18n/translations.js";
 import { getAllUserSettings, sendShoutrrrNotification, type UserSettings } from "../routes/settings.js";
@@ -25,7 +26,7 @@ import { updateReminderSentTime, updateUserReminderSentTime } from "./reminder-s
 const REMINDER_MINUTES_BEFORE = parseInt(process.env.REMINDER_MINUTES_BEFORE ?? "15", 10);
 const CHECK_INTERVAL_MS = 60 * 1000; // Check every 1 minute
 
-const intakeReminderStateFile = resolve(process.cwd(), "data", "intake-reminder-state.json");
+const intakeReminderStateFile = resolve(getDataDir(), "intake-reminder-state.json");
 
 function loadIntakeReminderState(): IntakeReminderState {
 	try {

backend/src/services/reminder-scheduler.ts

@@ -3,6 +3,7 @@ import { resolve } from "node:path";
 import { eq } from "drizzle-orm";
 import nodemailer from "nodemailer";
 import { db } from "../db/client.js";
+import { getDataDir } from "../db/db-utils.js";
 import { medications, userSettings } from "../db/schema.js";
 import { getTranslations, type Language, t } from "../i18n/translations.js";
 import { getAllUserSettings, sendShoutrrrNotification, type UserSettings } from "../routes/settings.js";
@@ -25,7 +26,7 @@ import {
 
 const REMINDER_HOUR = parseInt(process.env.REMINDER_HOUR ?? "6", 10); // Default 6:00 AM local time
 
-const reminderStateFile = resolve(process.cwd(), "data", "reminder-state.json");
+const reminderStateFile = resolve(getDataDir(), "reminder-state.json");
 
 function loadReminderState(): ReminderState {
 	try {