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>

docker-compose.example.yml

@@ -26,13 +26,14 @@ services:
       - "443:443/udp" # HTTP/3 (QUIC)
     volumes:
       - sealshare_storage:/app/storage/app   # Uploaded & encrypted files
-      - sealshare_database:/app/database      # SQLite database
+      - sealshare_database:/app/database/sqlite  # SQLite database
       - caddy_data:/data                      # TLS certificates
       - caddy_config:/config                  # Caddy configuration
     environment:
       # --- REQUIRED ---
       APP_URL:              # Your full URL, e.g. https://share.example.com
       # APP_KEY:            # Auto-generated if not set. Copy from logs to persist across restarts.
+      DB_DATABASE: /app/database/sqlite/database.sqlite  # The SQLite file in sealshare_database
 
       # --- HTTPS ---
       # Files are encrypted in the uploader's browser, which browsers only allow over HTTPS (or on
@@ -51,7 +52,7 @@ services:
       # DB_CONNECTION: sqlite           # Options: sqlite, mysql, pgsql
       # DB_HOST:                        # Required for mysql/pgsql
       # DB_PORT:                        # Required for mysql/pgsql
-      # DB_DATABASE:                    # Required for mysql/pgsql
+      # DB_DATABASE:                    # For mysql/pgsql the database's name, in place of the SQLite file above
       # DB_USERNAME:                    # Required for mysql/pgsql
       # DB_PASSWORD:                    # Required for mysql/pgsql
 
@@ -84,10 +85,11 @@ services:
     entrypoint: ["php", "artisan", "schedule:work"]
     volumes:
       - sealshare_storage:/app/storage/app
-      - sealshare_database:/app/database
+      - sealshare_database:/app/database/sqlite
     environment:
       # APP_KEY:            # Same key as the app service above (auto-generated if not set)
       APP_URL:              # Same URL as the app service above
+      DB_DATABASE: /app/database/sqlite/database.sqlite  # Same as the app service above
     depends_on:
       app:
         condition: service_healthy