docs: add indexer/realtime operational runbook (#849)

[extolkom]

Summary

Closes #849

Adds an operational runbook for the indexer/realtime system, covering
how on-call should read indexer lag, when to use reset vs replay
(including why replay is safe to re-run), and what the indexer's
environment variables control — including behavior when the indexer
is disabled. Pure documentation, no code changes.

What's included

backend/docs/SSE_ARCHITECTURE.md

New operational/runbook section covering:

• Reading indexer lag — from /health ([actual field names])
  and /v1/admin/metrics ([actual fields, if any beyond /health]),
  with the actual threshold values found in code ([threshold values]) and what each level means for on-call action.
• Reset vs replay — what each admin endpoint actually does
  ([route paths]), when to use which, and the idempotency guarantee
  for replay grounded in the dedup constraint at
  [file:line or constraint name].
• Env vars — INDEXER_POLL_INTERVAL_MS, INDEXER_START_LEDGER,
  STREAM_CONTRACT_ID, SOROBAN_RPC_URL: what each controls, and the
  exact resulting behavior when the indexer is disabled ([actual behavior found in code]).
• RPC-outage recovery walkthrough — step-by-step sequence for
  on-call when Soroban RPC goes down or ledgers are missed, referencing
  the reset/replay endpoints above.

backend/SSE_README.md

Short pointer section linking to the full runbook in
SSE_ARCHITECTURE.md, without duplicating content.

Scope / non-goals

• Does not re-document on-chain data flow (already covered in
  ARCHITECTURE.md)
• No code changes — documentation only
• [If Step 9's stop condition triggered on anything: note here what
  couldn't be located in code — e.g. no explicit lag threshold logic
  found — and how the doc handles that gap without inventing a number]

Grounding

Every claim in this doc (endpoint behavior, threshold values, env var
effects, idempotency mechanism) was verified directly against the
current implementation in backend/src/, not inferred from the issue
description alone. Specific references:

• Lag/health fields: [file path]
• Reset endpoint: [file path]
• Replay endpoint + dedup constraint: [file path]
• Env var handling / disabled-indexer behavior: [file path]

Done-when checklist (from #849)

• Lag reading + thresholds documented
• Reset/replay endpoints documented, including the idempotency note
• Env vars and disabled-indexer behavior documented

Verification

• Pure docs change — no build/test/lint impact expected, but ran:
  • [lint/format command if repo has one for markdown] — [pass/fail]
• Branch rebased on latest origin/main

Notes for reviewers

[Anything worth flagging — e.g. if a threshold or behavior found in
code seemed surprising or worth a maintainer double-check before this
becomes the on-call reference]

[extolkom]

The CI errors are pre-existing issues on the repository's main branch (LabsCrypt/flowfi) and are unrelated to the documentation PR: