chore(config): GitHub to Gitea is the default, every other host is beta (#386)

The GitLab and Gitea / Forgejo sources and the Forgejo, GitHub and GitLab destinations show a beta pill and beta wording; the docs say GitHub to Gitea is the supported path. Testing every source and destination pair against real hosts turned up three bugs, fixed here: the pull mirror path never recorded a row's destination host, reconcile counted rows mirrored to a previous destination as missing, and the first push into a freshly created GitLab project failed because the host reports it as not found for a couple of seconds.

docs/API.md

@@ -144,6 +144,7 @@ Nothing is deleted or archived on either side by this call; the cleanup service
     "notManaged": [{ "location": "me/notes", "reason": "not a mirror" }],
     "unverified": [],
     "healthyCount": 41,
+    "elsewhereCount": 0,
     "scannedOwners": ["e2e_admin", "github-mirrors"],
     "skippedOwners": ["starred"],
     "totalOnDestination": 43