chore(ecstore): collapse the expiry worker knobs to one documented env var (#6034)

init_background_expiry resolved its worker count through three env vars, none documented, none set in any known deployment: RUSTFS_MAX_EXPIRY_WORKERS, silently overridden by the underscore-prefixed _RUSTFS_ILM_EXPIRATION_WORKERS (a MinIO fossil, comment included), with a zero value then falling through to RUSTFS_DEFAULT_EXPIRY_WORKERS.

RUSTFS_MAX_EXPIRY_WORKERS stays as the canonical name per the rc constraint (no new env var names): the count is now resolved once — a set, parseable, non-zero value wins, anything else falls back to min(cpus, 16). The constant moves to rustfs-config's runtime constants alongside ENV_TRANSITION_WORKERS, the two dead names are gone repo-wide (rg-verified), and a serial four-state unit test (unset/zero/valid/garbage) pins the resolution, modeled on the transition-worker env harness.

Ref rustfs/backlog#1832 (PR2).

crates/config/src/constants/runtime.rs

@@ -81,6 +81,9 @@ pub const ENV_TEST_IAM_FAIL_INIT_ATTEMPTS: &str = "RUSTFS_TEST_IAM_FAIL_INIT_ATT
 pub const ENV_TEST_IAM_RETRY_INTERVAL_MS: &str = "RUSTFS_TEST_IAM_RETRY_INTERVAL_MS";
 /// Runtime env var controlling the transition worker count.
 pub const ENV_TRANSITION_WORKERS: &str = "RUSTFS_MAX_TRANSITION_WORKERS";
+/// Runtime env var controlling the ILM expiry worker count. A set, parsable,
+/// non-zero value wins; anything else falls back to `min(cpus, 16)`.
+pub const ENV_MAX_EXPIRY_WORKERS: &str = "RUSTFS_MAX_EXPIRY_WORKERS";
 /// Runtime env var controlling the absolute maximum transition workers.
 pub const ENV_TRANSITION_WORKERS_ABSOLUTE_MAX: &str = "RUSTFS_ABSOLUTE_MAX_WORKERS";
 /// Runtime env var controlling the transition queue capacity.