M6.5: matrix-synapse canonical Drone run #51 success (4 recipes now green via pipeline)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

BACKLOG.md

@@ -87,7 +87,7 @@ Two single-writer sections (§6.1): Builder edits only `## Build backlog`; Adver
 - [x] matrix-synapse (DB+media/large-volume, recipe #4) full 3-stage green on host: install 2✓
       (client API + versions JSON), upgrade 1✓ (postgres marker survives), backup 1✓ — exercises the
       recipe's pg_backup.sh DB-dump hook (not a plain volume copy). No harness surgery. Drone
-      canonical run = build #51 (in flight).
+      canonical run = **build #51 success** (~10.5m, all 3 stages green, clean teardown).
 - [ ] Enroll recipes 5–6: multi-service+S3 (lasuite-docs) + TLS-passthrough (bluesky-pds),
       no harness surgery
 - [ ] Re-verify keycloak backup post set_env fix (build #39 ran off an earlier backupbot deploy)