Fail open on clan tag ownership checks when API is unavailable

The clan-tag ownership check previously failed closed: when the API
service was unreachable (e.g. during local development), the client
dropped the tag with a "couldn't verify" error and the server's
FailOpenPrivilegeChecker treated every unverifiable tag as reserved.
This made clan tags unusable whenever the API was down.

- Client: checkClanTagOwnership keeps the tag when the existence
  probe is inconclusive; the server still re-checks authoritatively.
- Server: FailOpenPrivilegeChecker passes tags through instead of
  dropping non-member tags; decideClanTag now takes a non-nullable
  reserved set since the null case is gone.
- Remove the now-unused username.tag_check_failed translation key.
- Update Privilege and ClanApiQueries tests for fail-open behavior.

Trade-off: if the reserved-tag list is unavailable in production,
real clan tags can be impersonated until the first successful
PrivilegeRefresher load; after that the last good checker is retained.

resources/lang/en.json

@@ -682,8 +682,7 @@
     "tag_too_short": "Clan tag must be 2-5 alphanumeric characters.",
     "tag_too_long": "Clan tag cannot exceed 5 characters.",
     "tag_invalid_chars": "Clan tag can only contain letters and numbers.",
-    "tag_not_member": "Join the {tag} clan before using its tag. Click this message to join it.",
-    "tag_check_failed": "Couldn't verify clan tag. Try again or remove it."
+    "tag_not_member": "Join the {tag} clan before using its tag. Click this message to join it."
   },
   "host_modal": {
     "title": "Create Private Lobby",