api/v2: soft-revoke auth keys with a configurable collector

Tailscale's keys API has no separate expire verb: DELETE is the revoke.
Map it to a soft revoke so the key stays retrievable as invalid afterwards
instead of vanishing, matching the SDK and Terraform's expectations.

Add a revoked timestamp to pre-auth keys (migration plus schema), mark a
key invalid once revoked, and have the keys DELETE handler stamp it rather
than destroy the row. A background collector reaps revoked keys after a
configurable retention window (preauth_keys.revoked_retention, default
168h), so the table does not grow without bound.

hscontrol/db/schema.sql

@@ -50,6 +50,7 @@ CREATE TABLE pre_auth_keys(
   used numeric DEFAULT false,
   tags text,
   expiration datetime,
+  revoked datetime,
 
   created_at datetime,