fix(notifications): clamp size param to max 100 on GET /api/notifications

NullX Finding 2: unbounded size param allowed full table scan. Added
spring-boot-starter-validation, @Validated on the controller, @Min(1) @Max(100)
on the size param, and ConstraintViolationException → 400 in GlobalExceptionHandler.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/pom.xml

@@ -34,6 +34,10 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-actuator</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-validation</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-data-jpa</artifactId>