Chore: bump Docker index version to 1 (Tantivy); add language change detection

- Reset index_version to 1 — Tantivy is a full format change so
  versioning restarts from scratch; all existing v9 installs trigger
  an automatic reindex on next container start
- Add PAPERLESS_SEARCH_LANGUAGE change detection: track raw env var in
  .index_language so changing the language setting auto-reindexes;
  raw env var (not resolved language) avoids false positives from
  OCR_LANGUAGE inference
- docs/administration.md: clarify that Docker handles the post-upgrade
  reindex automatically; bare metal users need to run
  document_index reindex manually; add that as step 4 in the
  bare metal upgrade guide

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker/rootfs/etc/s6-overlay/s6-rc.d/init-search-index/run

@@ -3,9 +3,14 @@
 
 declare -r log_prefix="[init-index]"
 
-declare -r index_version=9
+# Version 1: Tantivy backend (replaces Whoosh; resets versioning from scratch)
+declare -r index_version=1
 declare -r data_dir="${PAPERLESS_DATA_DIR:-/usr/src/paperless/data}"
 declare -r index_version_file="${data_dir}/.index_version"
+declare -r index_language_file="${data_dir}/.index_language"
+# Track the raw env var (not the resolved language) so inference changes
+# don't cause spurious reindexes — only explicit setting changes trigger one.
+declare -r search_language="${PAPERLESS_SEARCH_LANGUAGE:-}"
 
 update_index () {
 	echo "${log_prefix} Search index out of date. Updating..."
@@ -13,16 +18,24 @@ update_index () {
 	if [[ -n "${USER_IS_NON_ROOT}" ]]; then
 		python3 manage.py document_index reindex --no-progress-bar
 		echo ${index_version} | tee "${index_version_file}" > /dev/null
+		echo "${search_language}" | tee "${index_language_file}" > /dev/null
 	else
 		s6-setuidgid paperless python3 manage.py document_index reindex --no-progress-bar
 		echo ${index_version} | s6-setuidgid paperless tee "${index_version_file}" > /dev/null
+		echo "${search_language}" | s6-setuidgid paperless tee "${index_language_file}" > /dev/null
 	fi
 }
 
-if [[ (! -f "${index_version_file}") ]]; then
+if [[ ! -f "${index_version_file}" ]]; then
 	echo "${log_prefix} No index version file found"
 	update_index
-elif [[ $(<"${index_version_file}") != "$index_version" ]]; then
-	echo "${log_prefix} index version updated"
+elif [[ $(<"${index_version_file}") != "${index_version}" ]]; then
+	echo "${log_prefix} Index version updated"
+	update_index
+elif [[ ! -f "${index_language_file}" ]]; then
+	echo "${log_prefix} No language file found"
+	update_index
+elif [[ $(<"${index_language_file}") != "${search_language}" ]]; then
+	echo "${log_prefix} Search language changed"
 	update_index
 fi