Stop the SQLite volume from hiding new migrations

Uploads on share.kadenpartner.ch failed with 409 on every chunk after
the update to 2.1. The example docker-compose.yml mounts the SQLite
volume over all of /app/database. Docker fills a volume from the image
only when it is created, so the container kept the 2.0 migrations and
never saw the 2.1 one. share_files.uploaded_chunks was never created,
so the chunk endpoint read null and answered 409. SQLite takes the
unknown "completed_at" in whereNull() as a string, so nothing failed
earlier.

- The image keeps a copy of its migrations in docker/migrations. On
  startup the entrypoint adds the ones missing from database/migrations
  before migrating, so installs with the old mount recover by pulling
  the new image.
- docker-compose.example.yml and docker-compose.yml mount
  sealshare_database at /app/database/sqlite and set DB_DATABASE to the
  file in it. An existing volume can be moved there without losing
  data: its database.sqlite lands at exactly that path.
- Tested with a locally built image: a volume created by 2.0.1 with the
  old mount gets the migration once (not again on restart) and keeps
  its settings; the same volume moved to the new path keeps its data;
  a fresh volume with the new layout starts healthy.
- README and CHANGELOG (Unreleased) describe the new path and how to
  switch.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,16 @@ All notable changes to this project are documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- Docker installs updated from 2.0 answered every upload with "409 Conflict". The example `docker-compose.yml` mounted the SQLite volume over all of `/app/database`, which hid the image's new migration, so it never ran. The container now adds the migrations the volume is missing before migrating, so existing compose files keep working.
+
+### Changed
+
+- `docker-compose.example.yml` mounts `sealshare_database` at `/app/database/sqlite` and sets `DB_DATABASE` to the file in it. To switch an existing install, mount the same volume there and set `DB_DATABASE: /app/database/sqlite/database.sqlite` in both services; the database is kept.
+
 ## [2.1.0] - 2026-09-16
 
 ### Added