Switch to master branch + Gitea Actions

- Move CI/release workflows from .github/workflows to .gitea/workflows (Gitea Actions),
  targeting the master branch.
- docker-publish: push to the Gitea container registry (git.finalfactory.de) with a
  lowercased image name; login via github.token or a PACKAGES_TOKEN secret.
- version-tag: gitea-actions bot identity; optional RELEASE_TOKEN to re-trigger the image build.
- Update README/CLAUDE.md/Unraid template/build-and-push.ps1 to the Gitea registry + master.

Claude-Session: https://claude.ai/code/session_01WujdMtMJPbxDpDnMeK22rr

deploy/build-and-push.ps1

@@ -1,10 +1,10 @@
 #!/usr/bin/env pwsh
 # Local convenience: build the multi-arch MeterVault image and push it.
-# Usage:  ./deploy/build-and-push.ps1 -Registry ghcr.io -Image finalfactory/metervault
+# Usage:  ./deploy/build-and-push.ps1 -Registry git.finalfactory.de -Image finalfactory/metervault -Push
 # Version is read from the VERSION file; images are tagged :<version> and :latest.
 
 param(
-    [string]$Registry = "ghcr.io",
+    [string]$Registry = "git.finalfactory.de",
     [string]$Image = "finalfactory/metervault",
     [string[]]$Platforms = @("linux/amd64", "linux/arm64"),
     [switch]$Push