chore: harden common storage bootstrap and slim embedded db assets

Unify embedding config, build providers from system settings, and fail
startup when index builds error or time out. Move Surreal assets under
common/db so embeds exclude crate source, and read storage via streams.

common/db/migrations/20260528_000001_file_user_sha256_index.surql

@@ -0,0 +1,3 @@
+-- Per-user deduplication: same SHA256 may exist for different users.
+REMOVE INDEX IF EXISTS file_sha256_idx ON file;
+DEFINE INDEX IF NOT EXISTS file_user_sha256_idx ON file FIELDS user_id, sha256 UNIQUE;