docs(ops): add retention-gap recovery runbook

[Hustler490]

Closes #159

What

The deployment guide documents Soroban RPC retention windows and the mitigation options, but stops there. It never tells an operator what to actually do when a scanner is offline past the boundary. This adds a dedicated recovery runbook.

Changes

• guides/ops/retention-gap-recovery.mdx (new) - step-by-step procedure covering:
  • Detection - compare lastProcessedLedger to the network tip and classify the gap against the ~17,280-ledger window; confirm via retention_window_exceeded / RetentionExceededError.
  • Checkpoint handling - stop the scanner, capture lastProcessedLedger before and after, never advance the checkpoint past unpersisted events.
  • Archive replay - full-history node with fallbackPolicy: "archive_node".
  • Dual-provider recovery - fail over to an independent provider for the missed range.
  • Horizon fallback - best-effort reconstruction when no archive/secondary covers the range.
  • Safe rescan + deduplication - stable event identity (contract_id, ledger, tx_hash, event_index) with a unique constraint and ON CONFLICT DO NOTHING upsert.
  • Alert thresholds - approaching-boundary (medium) and past-boundary (critical) Prometheus rules.
  • Operator evidence checklist and a post-recovery verification list.
• guides/ops/monitoring-and-on-call.mdx - links the runbook from Playbook 3 (Indexer Stall) and Cross-References.
• docs.json - registers the page in the Operations navigation group.

Validation

• node localization-qa.cjs ? 0 issues (new page recognised as a canonical page).
• node scripts/check-nav-coverage.mjs ? 72 pages checked, 72 nav entries verified.
• docs.json parses as valid JSON.

[truthixify]

The replacement removes the fake CLI commands, but several examples still use APIs that do not exist. fetchAnnouncementsStream is a Stellar entry-point export and takes (chain, options); it has no contractIds, retention, or fallbackPolicy. Horizon has no getTransactions helper, and announcements do not expose event_index. Please align the runbook with the current SDK and existing stellar_network_ledger_height metric.