Defer promotion snapshots to campaign activation

src/lib/server/db/schema.sql

@@ -365,7 +365,9 @@ CREATE TABLE IF NOT EXISTS deployment_files (
     sha256_before  TEXT,
     sha256_after   TEXT,
     content_before BLOB,
-    content_after  BLOB
+    content_after  BLOB,
+    -- An unknown before image is distinct from a confirmed absent file.
+    snapshot_pending INTEGER NOT NULL DEFAULT 0
 );
 
 CREATE INDEX IF NOT EXISTS idx_deployment_files_dep ON deployment_files(deployment_id);