chore: add SPDX copyright headers and update license year

Add `// Copyright Sunbeam Studios 2026` and `// SPDX-License-Identifier:
Apache-2.0` headers to all source files missing them. Update LICENSE
copyright year, Dockerfile copyright header, and .dockerignore for new
project structure (lean4/, docs/, training artifacts).

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

tests/rate_limit_test.rs

@@ -1,3 +1,6 @@
+// Copyright Sunbeam Studios 2026
+// SPDX-License-Identifier: Apache-2.0
+
 use std::net::IpAddr;
 use sunbeam_proxy::config::{BucketConfig, RateLimitConfig};
 use sunbeam_proxy::rate_limit::key::{self, RateLimitKey};