feat(auth): add detailed logging for login and 2FA operations

Introduce warnings for failed login attempts and invalid 2FA tokens. Add logs for successful logins and 2FA requirements to improve authentication traceability.

backend/src/auth/auth.service.ts

@@ -110,6 +110,7 @@ export class AuthService {
 		const user = await this.usersService.findByEmailHash(emailHash);
 
 		if (!user) {
+			this.logger.warn(`Login failed: user not found for email hash`);
 			throw new UnauthorizedException("Invalid credentials");
 		}
 
@@ -119,10 +120,12 @@ export class AuthService {
 		);
 
 		if (!isPasswordValid) {
+			this.logger.warn(`Login failed: invalid password for user ${user.uuid}`);
 			throw new UnauthorizedException("Invalid credentials");
 		}
 
 		if (user.isTwoFactorEnabled) {
+			this.logger.log(`2FA required for user ${user.uuid}`);
 			return {
 				message: "2FA required",
 				requires2FA: true,
@@ -141,6 +144,7 @@ export class AuthService {
 			ip,
 		);
 
+		this.logger.log(`User ${user.uuid} logged in successfully`);
 		return {
 			message: "User logged in successfully",
 			access_token: accessToken,
@@ -165,6 +169,7 @@ export class AuthService {
 
 		const isValid = authenticator.verify({ token, secret });
 		if (!isValid) {
+			this.logger.warn(`2FA verification failed for user ${userId}: invalid token`);
 			throw new UnauthorizedException("Invalid 2FA token");
 		}
 
@@ -179,6 +184,7 @@ export class AuthService {
 			ip,
 		);
 
+		this.logger.log(`User ${userId} logged in successfully via 2FA`);
 		return {
 			message: "User logged in successfully (2FA)",
 			access_token: accessToken,