feat(llm): add configurable request/token rate limiting for LLM client

Introduce process-local and Redis-backed global rate limiting for the
shared LLM client, plus a dedicated generation consumer prefetch knob.

- Add request (RPM) and token (TPM) rate limits with burst controls,
  process or global scope, and fail-closed/process-degraded failover
- Wire the LLM client through NewWithRedis so limits can be shared
  across replicas via a Redis token bucket
- Add generation_consumer_prefetch to tune RabbitMQ generation
  consumers independently of the default prefetch
- Bump generation worker_concurrency to 24 and promote golang.org/x/time
  to a direct dependency
- Drop t.Parallel() from a few middleware/bootstrap tests

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/go.mod

@@ -33,6 +33,7 @@ require (
 	golang.org/x/image v0.38.0
 	golang.org/x/net v0.48.0
 	golang.org/x/sync v0.20.0
+	golang.org/x/time v0.5.0
 	gopkg.in/yaml.v3 v3.0.1
 )
 
@@ -83,7 +84,6 @@ require (
 	golang.org/x/arch v0.3.0 // indirect
 	golang.org/x/sys v0.39.0 // indirect
 	golang.org/x/text v0.35.0 // indirect
-	golang.org/x/time v0.5.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217 // indirect
 	google.golang.org/grpc v1.79.3 // indirect
 	google.golang.org/protobuf v1.36.10 // indirect