feat(server): add Audiobookshelf and Calibre-Web Automated migration sources

Extend the migration module beyond Booklore and Grimmory so an existing Audiobookshelf or
Calibre-Web Automated library can be moved into BookOrbit without rebuilding user state by
hand. Audiobookshelf supports both a live API connection and an offline backup file, while
Calibre-Web Automated reads a stopped snapshot of the app and metadata databases.

Match source books by ASIN in addition to ISBN, and batch the ISBN, ASIN, file hash, file
path, and title/author lookups so planning a large library issues a bounded number of
queries rather than one per book. Route the batched author comparison through the shared
accent-insensitive predicate, which now accepts a SQL fragment so a per-row pattern column
is matched the same way a bound literal is.

Extract the shared source connection form into MigrationSourceFields so the modal and the
settings page stay in sync as source types are added, and allow a mapping to skip a source
user instead of forcing every account to be imported.

Closes #986

.env.example

@@ -53,6 +53,10 @@ NODE_MAX_OLD_SPACE_SIZE=2048
 # Recommended if you store migration source credentials (generate: openssl rand -hex 32)
 # MIGRATION_ENCRYPTION_KEY=
 
+# Absolute container path that authorizes local migration backup files.
+# Mount a dedicated host directory at this path as read-only before enabling backup mode.
+# MIGRATION_IMPORT_ROOT=/imports
+
 # Log level (default: info). Set to debug for verbose output.
 # LOG_LEVEL=info