feat: rate limiting + E2E crypto lifecycle test

Rate limiting (closes the last security gap #3):
- New RateLimiter: in-memory fixed-window IP-based limiter (std::sync::Mutex
  HashMap, no new deps). Configurable via RATE_LIMIT_MAX (default 100) +
  RATE_LIMIT_WINDOW_SECS (default 60) env vars.
- general_rate_limit_middleware now reads ClientIp from request extensions and
  rejects with 429 + Retry-After header when over the limit. Wired via
  from_fn_with_state in app.rs. Lived on AppState as Arc<RateLimiter>.
- Deleted the dead auth_rate_limit_middleware (never wired).
- 3 unit tests (allows up to N, independent IPs, window reset).

E2E crypto lifecycle test:
- Full zero-knowledge round-trip against jsdom's real Web Crypto: setup →
  encrypt → verify ciphertext → unlock with password → decrypt → recover via
  phrase → rewrap under new password → decrypt. Plus wrong-password/wrong-phrase
  failures and cross-user key isolation.
- Fixed wrapDek/unwrapDek: base64-encode raw DEK bytes (was using TextDecoder
  which produced non-base64), and make unwrapped DEK extractable (needed for
  rewrapDek to export).

Verified: backend 24 tests 0 warnings; frontend 24 tests, build clean.

backend/tests/common/mod.rs

@@ -142,6 +142,10 @@ pub async fn app_for_test() -> Option<(Router, String)> {
         interaction_service: Some(interaction_service),
         token_version_cache,
         refresh_token_repo: Some(refresh_token_repo),
+        rate_limiter: std::sync::Arc::new(normogen_backend::security::RateLimiter::new(
+            1000,
+            std::time::Duration::from_secs(60),
+        )),
     };
 
     Some((build_app(state), db_name))