fix(curation): blacklist source_contexts of un-startable backfill seeds

Oracle round-6 watch-out closure. The Assistant's list_processes was
returning PO Approval + Purchase Order Approval, both of which return
500 transaction_creation_failed on /api/runtime/transactions because
they were backfilled into the catalogue without view/data attachments.

Added 5 source_context exclusions matching the seed-time markers of
those flows: D2 live backfill / EA2 runtime reference seed /
atlas-f1-fresh-proof / EA2 seed payload via POST /api/process /
fm-process-mining-conformance-reseed.

The canonical pr_to_po_def (source_context fm06-t10-demo-reset-v2,
display 'Purchase Requisition to PO') survives — it is the only
runtime-startable flow in the tenant today.

src/lib/flowCuration.ts

@@ -40,6 +40,11 @@ const NON_BUSINESS_SOURCE_CONTEXTS = new Set([
   "CANVAS_SEED_STEP",
   "EA2_WIZARD_STEP",
   "EA2_DRAFT_PROCESS:process_creation",
+  "D2 live backfill 2026-04-25; derived from seed fields, existing description/content, and graph name.",
+  "EA2 runtime reference seed",
+  "atlas-f1-fresh-proof",
+  "EA2 seed payload via POST /api/process",
+  "fm-process-mining-conformance-reseed",
 ]);
 
 const HEX32 = /[a-f0-9]{32}/gi;