fix(auth): bump default API host to v2-13-10 (#44)

The previous bump landed on v2-12-5, which the probe picked because it
didn't return "server is deprecated". v2-12-5 turned out to return 500
"database is unreachable" for every call — not actually a working host.

Manual sweep across the full v2-13-x range (anchored on the current
iOS app version 2.13.10, released 2026-05-08) shows v2-13-6 through
v2-13-10 are healthy: they return "Session not found" for an
invalid-token probe, which is the expected 4xx-style response from a
working backend. v2-13-5 and below are deprecated.

v2-13-10 verified against a live refresh:
  $ LIFTOFF_API_BASE=https://v2-13-10.api.getgymbros.com \
      liftoff-export auth refresh
  Token valid: eyJhbGciOiJIUzI1NiIs...

The probe will get a separate PR teaching it to distinguish "live"
(healthy 4xx) from "broken/transient" (500) from "deprecated", so it
doesn't fall into the same trap on the next rotation.

Co-authored-by: Orca <[email protected]>

Author: DTTerastar

internal/auth/auth.go

@@ -15,7 +15,7 @@ import (
 )
 
 const (
-	defaultAPIBase   = "https://v2-12-5.api.getgymbros.com"
+	defaultAPIBase   = "https://v2-13-10.api.getgymbros.com"
 	refreshProcedure = "user.refreshToken"
 	userAgent        = "Liftoff/528 CFNetwork/3860.400.51 Darwin/25.3.0"
 )