feat: add rate limiting and job queue depth cap (closes #15)

- Add custom RateLimitMiddleware using governor crate for per-IP rate limiting
- Two-tier rate limiting: destructive (20 req/min, burst 10) and read (120 req/min, burst 30)
- Health endpoints (/health, /api/v1/system/info) exempt from rate limiting
- Add max_queue_depth to JobManager (default: 100, configurable via config.yaml)
- Return 429 Too Many Requests with Retry-After header when queue is full
- Add RateLimitConfig to config.yaml with all rate limit settings
- Add 10 tests covering rate limiting, queue depth, and configuration defaults

Cargo.toml

@@ -16,6 +16,9 @@ actix-web-actors = "4"
 actix = "0.13"
 actix-tls = { version = "3", features = ["rustls-0_23"] }
 
+# Rate limiting (actix-governor for per-IP rate limiting)
+actix-governor = "0.6"
+
 # Async runtime
 tokio = { version = "1", features = ["full"] }
 
@@ -118,6 +121,10 @@ path = "tests/e2e/test_enrollment_e2e.rs"
 name = "auth_test"
 path = "tests/integration/auth_test.rs"
 
+[[test]]
+name = "rate_limit_test"
+path = "tests/unit/rate_limit_test.rs"
+
 [[bench]]
 name = "api_benchmarks"
 harness = false