Feature: run pending LLM index migrations automatically on startup

Add document_llmindex migrate, a cheap check-only path (no reindex) safe
to run unconditionally: has_pending_migration() short-circuits to a
metadata-only read once the store is current, so a healthy install pays
almost nothing. If a pending migration would require re-embedding, it
only logs a warning and leaves the index as-is -- re-embedding can be
slow and, for a metered embedding backend, cost money, so it stays a
deliberate manual action (document_llmindex rebuild), never automatic.

Wire it into the Docker image as a new init-llmindex-migrate oneshot
(modeled on init-search-index, gated on init-migrations), and document
the equivalent manual step for bare-metal upgrades.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker/rootfs/etc/s6-overlay/s6-rc.d/init-llmindex-migrate/run

@@ -0,0 +1,12 @@
+#!/command/with-contenv /usr/bin/bash
+# shellcheck shell=bash
+
+declare -r log_prefix="[init-llmindex-migrate]"
+
+echo "${log_prefix} Checking LLM index schema..."
+cd "${PAPERLESS_SRC_DIR}"
+if [[ -n "${USER_IS_NON_ROOT}" ]]; then
+	python3 manage.py document_llmindex migrate
+else
+	s6-setuidgid paperless python3 manage.py document_llmindex migrate
+fi

docker/rootfs/etc/s6-overlay/s6-rc.d/init-llmindex-migrate/type

@@ -0,0 +1 @@
+oneshot

docker/rootfs/etc/s6-overlay/s6-rc.d/init-llmindex-migrate/up

@@ -0,0 +1 @@
+/etc/s6-overlay/s6-rc.d/init-llmindex-migrate/run