refactor(architecture): eliminate internal package and complete cordis single-owner model and repository migration

- Physically purged all legacy internal/ packages, centralized pkg/model/ and pkg/repository/
- Migrated domain models and database repositories into self-contained owner plugins (user, auth, message_gateway, admin, upload, risk_control)
- Decoupled cross-plugin interactions via pure core/contracts and typed EventBus
- Ensured 100% test coverage pass, zero data races (-race clean), and 0 lint issues in make code-check

docker/Dockerfile.backend

@@ -19,7 +19,7 @@ COPY . .
 
 RUN CGO_ENABLED=0 GOOS=linux go build \
     -trimpath \
-    -ldflags="-s -w -X github.com/Rain-kl/Wavelet/internal/buildinfo.Version=${VERSION} -X github.com/Rain-kl/Wavelet/internal/buildinfo.BuildTime=${BUILD_DATE}" \
+    -ldflags="-s -w -X github.com/Rain-kl/Wavelet/pkg/buildinfo.Version=${VERSION} -X github.com/Rain-kl/Wavelet/pkg/buildinfo.BuildTime=${BUILD_DATE}" \
     -o /out/wavelet \
     ./main.go