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>

docs/administration.md

@@ -180,6 +180,17 @@ following:
     This might not actually do anything. Not every new paperless version
     comes with new database migrations.
 
+4.  Rebuild the search index.
+
+    ```shell-session
+    cd src
+    python3 manage.py document_index reindex
+    ```
+
+    This is required when the search backend has changed (e.g. the upgrade
+    to Tantivy). It is safe to run on every upgrade — if the index is already
+    current it completes quickly.
+
 ### Database Upgrades
 
 Paperless-ngx is compatible with Django-supported versions of PostgreSQL and MariaDB and it is generally
@@ -469,9 +480,20 @@ task scheduler.
 
 !!! note
 
-    On first startup after upgrading from a previous version, paperless detects
-    that the index format has changed and automatically performs a one-time full
-    reindex. No manual migration step is required.
+    **Docker users:** On first startup after upgrading, the container automatically
+    detects the index format change and runs a full reindex before starting the
+    webserver. No manual step is required.
+
+    **Bare metal users:** After upgrading, run the following command once to rebuild
+    the search index in the new format:
+
+    ```shell-session
+    cd src
+    python3 manage.py document_index reindex
+    ```
+
+    Changing `PAPERLESS_SEARCH_LANGUAGE` also requires a manual reindex on bare
+    metal (Docker handles this automatically).
 
 ### Clearing the database read cache