docs(readme): use Maven wrapper instead of bare mvn

No system Maven is installed; backend/mvnw is the correct entry point.
Updated run and verify-everything commands accordingly.

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

README.md

@@ -35,7 +35,7 @@ Open two terminals:
 
 ```bash
 # Backend → http://localhost:8080
-mvn -f backend/pom.xml spring-boot:run
+backend/mvnw -f backend/pom.xml spring-boot:run
 
 # Frontend → http://localhost:5173  (proxies /api → :8080)
 cd frontend && npm run dev
@@ -50,9 +50,9 @@ mise run check       # format-check + lint + test (both stacks)
 Without mise:
 
 ```bash
-mvn -f backend/pom.xml spotless:check -q
+backend/mvnw -f backend/pom.xml spotless:check -q
 cd frontend && npx biome check src
-mvn -f backend/pom.xml test -q
+backend/mvnw -f backend/pom.xml test -q
 cd frontend && npx vitest run
 ```