Add backend API for projects & snapshots

Introduce full backend API surface for projects, snapshots and user pubkeys: controllers, DTOs, JPA entities, repositories, services, and tests. Add SecurityConfig improvements (JWT decoder, audience/issuer/timestamp validation and CORS), a GlobalExceptionHandler for consistent error responses, and remove a stray test call from BackendApplication. Also update README (architecture, docs, CLI/Frontend/Backend notes), add CLAUDE.md, VSCode settings, and multiple frontend/CLI Rust changes. Small housekeeping: add .DS_Store and test adjustments.

.vscode/settings.json

@@ -0,0 +1,3 @@
+{
+    "java.compile.nullAnalysis.mode": "automatic"
+}