feat: Rust refactor — full backend rewrite (Phases 1-6)

Phase 1-3: Scaffold + types + auth (JWT, Argon2, API keys)
Phase 4a: Admin API (sites/accounts/tokens/downstream-keys/settings)
Phase 5: Proxy core (/v1/chat, /v1/images, /v1/models)
Phase 6: ComfyUI proxy routes migrated

- 80+ source files, all cargo check + 18 tests green
- Drop-in compatible with Node.js API surface

boosapi-rs/.gitignore

@@ -0,0 +1,5 @@
+/target
+**/*.rs.bk
+Cargo.lock
+.env
+data/