ios: get TestFlight CI signing working

Replace the old testflight-release workflow with a single
`testflight.yml` Gitea Actions workflow and rework the fastlane `beta`
lane: match-based app-store signing into a disposable CI keychain,
XcodeGen project generation, version/build-number bumping, and upload
to TestFlight. Pin Ruby to 3.1.7 for the runner.

Squashes the iterative CI signing debugging history into one commit.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ios/fastlane/Appfile

@@ -1,9 +0,0 @@
-require "dotenv"
-
-Dotenv.load(File.expand_path("../.env", __dir__))
-
-app_identifier(ENV.fetch("FASTLANE_APP_IDENTIFIER", "net.buzzert.sybil2"))
-team_id(ENV.fetch("FASTLANE_TEAM_ID", "DQQH5H6GBD"))
-
-apple_id(ENV["FASTLANE_USER"]) if ENV["FASTLANE_USER"].to_s.strip.length.positive?
-itc_team_id(ENV["FASTLANE_ITC_TEAM_ID"]) if ENV["FASTLANE_ITC_TEAM_ID"].to_s.strip.length.positive?