feat: add permanent API keys for users

- Add api_key column to users table with auto-migration
- Modify auth middleware to accept both JWT (24h) and API keys (permanent)
- Add endpoints: GET/POST/DELETE /api/users/:id/api-key
- Update admin panel with API key management (generate, copy, regenerate, revoke)
- Update API documentation modal to show permanent API key usage

API keys use the format: nf_XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Use header: X-API-Key: <your-key>

config/database.js

@@ -97,10 +97,21 @@ async function initDatabase() {
         username VARCHAR(255) UNIQUE NOT NULL,
         password_hash VARCHAR(255) NOT NULL,
         is_admin BOOLEAN DEFAULT FALSE,
+        api_key VARCHAR(64) UNIQUE,
         created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
       )
     `);
 
+    // Migration: ajouter la colonne api_key si elle n'existe pas
+    await client.query(`
+      DO $$
+      BEGIN
+        IF NOT EXISTS (SELECT 1 FROM information_schema.columns WHERE table_name = 'users' AND column_name = 'api_key') THEN
+          ALTER TABLE users ADD COLUMN api_key VARCHAR(64) UNIQUE;
+        END IF;
+      END $$;
+    `);
+
     // Table notes
     await client.query(`
       CREATE TABLE IF NOT EXISTS notes (