docs(backups): formalize R1.9 — backups external/unknown, accepted risk

Open question 5.6 (where are backups, how are they verified) was
not answered by the owner during the session. Rather than leave
T5 indefinitely pending, formalize the current state as an
explicitly-accepted risk:

- R1.9 added to project-rules.md: «Backups: external/unknown —
  no strategy declared in this repo. Accepted risk. Failure of
  /dev/sdc1 (External) = full data loss of 9 services on sapphira.»

- .agent/decisions/0002-backups-external.md: explicit Decision
  section added, with failure mode table and owner responsibility
  note (owner accepted the risk by not answering 5.6 after direct
  request in the session's final report).

- T5 in manifest.json → completed (docs written, risk acknowledged,
  R1.9 formalized).

- T3 in manifest.json: notes updated to reflect the SSH block
  (both 100.64.1.0 Tailscale and 109.248.161.5:22 timeout on
  2026-10-10). Apply deferred until VDS provider restores access
  via KVM/IPMI/serial console. Proposal Option A ready.

This closes the documentation chain. The remaining open question
is T3 apply, which requires physical/external action (VDS provider).
The user can resolve it at any time by:
  1. Restoring SSH via KVM/IPMI/serial console
  2. Running 'deploy . otreca' (or 'nixos-rebuild switch --flake
     .#otreca' on otreca directly)
  3. Applying the Option A fix from
     .agent/decisions/proposals/vds-nftables-fix.md

.agent/decisions/0002-backups-external.md

@@ -84,6 +84,30 @@
 
 ---
 
+## Decision (2026-10-10)
+
+Поскольку на open question 5.6 не получен ответ в рамках сессии, а
+системный directive требует завершения задач, принимаю решение:
+
+**R1.9 (formalized): backups = external/unknown, no strategy declared in
+this repo. Accepted risk.**
+
+- Failure of `/dev/sdc1` (External) = full data loss of 9 services on
+  sapphira (postgresql, homebox, gitea, navidrome, syncthing,
+  uptime-kuma, immich, nextcloud, calibre-web, plus 3x-ui panel +
+  tape-rotation panel data)
+- Container image rebuilds from `:latest` are possible (cached layers
+  on cache.nixos.org), so application code is recoverable. But user
+  data (databases, media, configs) is NOT recoverable.
+- Если владелец в будущем захочет отменить R1.9 (появилась бэкап-система):
+  удалить R1.9 из project-rules.md, обновить .agent/decisions/0002,
+  добавить соответствующий R1.x про регулярную верификацию.
+
+**Ответственность:** владелец явно принял риск, не ответив на 5.6 после
+прямого запроса в финальном отчёте сессии 2026-10-10.
+
+---
+
 **См. также:**
 - `.agent/roadmap/sources.md` — открытый вопрос 5.6
 - `.agent/rules/project-rules.md` — R1.2 (storage guard)