M6.5: matrix-synapse (recipe #4) full 3-stage green on host (postgres-marker DB-hook); Drone #51 in flight

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

BACKLOG.md

@@ -84,8 +84,12 @@ Two single-writer sections (§6.1): Builder edits only `## Build backlog`; Adver
       (handles cryptpad's SANDBOX_DOMAIN). Fixed a real backup bug en route: set_env glued
       RESTIC_REPOSITORY onto a comment → backupbot had no restic repo (now newline-safe). Drone
       canonical run = **build #46 success** (~6m, all 3 stages green, clean teardown).
-- [ ] Enroll recipes 4–6 covering remaining D10 categories (multi-service+S3, large-volume,
-      TLS-passthrough), no harness surgery
+- [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).
+- [ ] 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)
 - [ ] Gate: M6.5 — recipes 3–6 three-stage green