feat: configure PostgreSQL to work with local server

Successfully configured and connected PostgreSQL database for NoteFlow.

Backend changes:
- Added dotenv package and configured .env loading in server.js
- Increased PostgreSQL connection timeout from 2s to 10s
- Added connection URL logging for debugging
- Configured to use PostgreSQL on port 5432 (instead of 5499)

PostgreSQL setup:
- Installed PostgreSQL 16 locally
- Created 'noteflow' database and user
- Fixed SSL certificate permissions
- Configured TCP listening on localhost
- Set up password authentication (md5)

Database configuration:
- Uses .env file for DATABASE_URL and credentials
- Connection: postgresql://noteflow:***@localhost:5432/noteflow
- All tables created and initialized successfully
- Admin user created (username: admin, password: admin)

This resolves the "no notes displaying" issue. The application now
connects to a working PostgreSQL database and can store/retrieve notes.

Files changed:
- server.js: Added dotenv.config() at top
- config/database.js: Increased timeout, added logging
- package.json: Added dotenv dependency
- config/database-postgres.js: Backup of PostgreSQL config

server.js

@@ -1,4 +1,7 @@
 // Serveur Express principal pour l'application NoteFlow
+// Load environment variables from .env file
+require('dotenv').config();
+
 const express = require('express');
 const cors = require('cors');
 const helmet = require('helmet');