feat(lock): mark Exists as NO_SIDE_EFFECTS

Annotate the only read-only method on lock.v1.LockService with
idempotency_level = NO_SIDE_EFFECTS so Connect-RPC handlers accept
HTTP GET for it. Callers can probe lock state via a cacheable URL
without a POST round-trip.

Lock, LockRead, Release, ForceRelease, UpdateTTL stay POST-only —
they mutate the locker's internal map (even Lock with wait=0 creates
a resource entry on first reference).

Additive proto change; existing clients keep working.

Author: rustatian

roadrunner/api/lock/v1/service.proto

@@ -13,6 +13,8 @@ service LockService {
   rpc LockRead(LockRequest) returns (LockResponse);
   rpc Release(LockRequest) returns (LockResponse);
   rpc ForceRelease(LockRequest) returns (LockResponse);
-  rpc Exists(LockRequest) returns (LockResponse);
+  rpc Exists(LockRequest) returns (LockResponse) {
+    option idempotency_level = NO_SIDE_EFFECTS;
+  }
   rpc UpdateTTL(LockRequest) returns (LockResponse);
 }