Implement token bucket rate limiting with IPv6 /64 normalization

- Add TokenBucket with nodelay semantics (nginx limit_req burst nodelay)
- Per-IP rate limiting: IPv4 /32, IPv6 /64 prefix normalization
- DashMap for concurrent access, ArcSwap for lock-free config reads
- Background eviction task for stale entry cleanup
- 429 response with plain text body, RATE_LIMIT log prefix
- Config reload adopts new rate/burst on next request without clearing state
- Unit tests for bucket algorithm and IPv6 normalization
- Integration tests for 429 responses and per-IP independence

Cargo.lock

@@ -489,6 +489,20 @@ version = "0.8.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d0a5c400df2834b80a4c3327b3aad3a4c4cd4de0629063962b03235697506a28"
 
+[[package]]
+name = "dashmap"
+version = "6.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5041cc499144891f3790297212f32a74fb938e5136a14943f338ef9e0ae276cf"
+dependencies = [
+ "cfg-if",
+ "crossbeam-utils",
+ "hashbrown 0.14.5",
+ "lock_api",
+ "once_cell",
+ "parking_lot_core",
+]
+
 [[package]]
 name = "data-encoding"
 version = "2.11.0"
@@ -789,6 +803,12 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "hashbrown"
+version = "0.14.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e5274423e17b7c9fc20b6e7e208532f9b19825d82dfd615708b70edd83df41f1"
+
 [[package]]
 name = "hashbrown"
 version = "0.17.1"
@@ -1064,7 +1084,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d466e9454f08e4a911e14806c24e16fba1b4c121d1ea474396f396069cf949d9"
 dependencies = [
  "equivalent",
- "hashbrown",
+ "hashbrown 0.17.1",
 ]
 
 [[package]]
@@ -1584,6 +1604,7 @@ dependencies = [
  "arc-swap",
  "axum",
  "clap",
+ "dashmap",
  "futures",
  "hyper",
  "rcgen",