chore(ci): Adds nightly and release build workflows

- Introduces workflows for nightly and release builds,
  automating package publishing to Gitea.
- Includes scripts for setting Poetry version, cleaning up
  old dev versions, and uploading assets to releases.
- Adds a merge template for pull requests.

.gitea/scripts/set_poetry_version.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+BASE_VERSION=$(cat VERSION)
+NIGHTLY_SUFFIX=""
+
+if [[ "$1" == "nightly" ]]; then
+  # Beispiel: 20240511.1358 → 11. Mai, 13:58 Uhr
+  NIGHTLY_SUFFIX=".dev$(date +%Y%m%d%H%M)"
+fi
+
+FULL_VERSION="${BASE_VERSION}${NIGHTLY_SUFFIX}"
+
+echo "Using version: $FULL_VERSION"
+poetry version "$FULL_VERSION"