Un-key stage-cache rows written before the current key shape

CACHE_SHAPE_SINCE marks the last change that moved every cache key. The
run service's periodic sweep clears cache_key on rows of runs created
before it, which nothing can match any more. Their outputs stay.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

backend/fluksio/flow/controller.py

@@ -1216,7 +1216,8 @@ class FlowController:
                     # reaches hash to, which `sync` worked out by following
                     # its imports. The ports are here because declaring a new
                     # one changes what the node answers with, which a hit
-                    # would otherwise restore without.
+                    # would otherwise restore without. Changing what goes in
+                    # here changes every key: move `CACHE_SHAPE_SINCE` with it.
                     node.fingerprint = hashlib.sha256(
                         json.dumps(
                             {