Document sync_interval for deb/alpine and fix created_at gate on existing rows (#176)

`sync_interval` is documented only for github_rpm, and the claim gate holding back remotes created inside the current slot also blocks remotes that already have a sync state row: the gate filters the INSERT SELECT, so ON CONFLICT never runs and a failed cold-cache prime waits a full slot instead of its backoff.

- document `sync_interval` for github_deb/github_alpine and in the caching field table
- apply the created_at gate only when no sync state row exists
- test 0→N and N→0 interval changes and the insert-only gate against Postgres
- test that a negative `sync_interval` on update returns 400

Reviewed-on: #176
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/api/v2/remotes_ttl_test.go

@@ -101,6 +101,12 @@ func TestSyncIntervalAPI(t *testing.T) {
 			t.Errorf("%s = %d, want 400", body, c)
 		}
 	}
+	if c := do(t, h, "PUT", "/si-gh", `{`+gh+`,"sync_interval":-1}`); c != 400 {
+		t.Errorf("update negative = %d, want 400", c)
+	}
+	if got := interval("si-gh"); got != 3600 {
+		t.Errorf("rejected update changed sync_interval to %d", got)
+	}
 }
 
 func TestQuarantineDaysDefault(t *testing.T) {