TSA-495: Include rate-limited and retry charges in document_cost

[anjoanka]

Closes #495

Summary

document_cost only summed the transcription table, but the worker charges
document.spent_usd in two places where no transcription row is written:

• rate-limited repair call (deferRateLimitedPage): tokens from the first call
  are charged, the page is deferred, the charge lives only in
  page_event transcribe_rate_limited
• retryable failure (handleFailedTranscription with retry): the charge lives
  only in page_event transcribe_failed with transcription_id = NULL

Budget enforcement was correct, but the cost view understated spending — or
returned no row at all for a document stopped by its budget on such calls.

Changes

• New migration recreating document_cost: sums transcription rows plus
  page_event charges (transcribe_rate_limited / transcribe_failed) that have
  no transcription row and costUsd > 0. Events linked to a transcription row are
  skipped, so nothing is counted twice. Columns are unchanged; calls,
  avg_latency_ms and cache_hits still describe transcription rows only.
  down() restores the previous view.
• transcribe.ts: the transcribe_rate_limited event now also stores
  inputTokens / outputTokens, same as transcribe_failed already does.
• Docs: docs/schema/schema.sql and docs/04-database.md updated.

The retryable-failure path is not in the original issue, but it is the same
root cause in the same view, so it is fixed here too.

How to test

1. npm run migrate:dev -w apps/backend
2. Inside a transaction (rolled back at the end), insert a document with two
   rate-limited events ($0.000144 each) and a document with a retry charge
   ($0.003) + a final failure with a transcription row ($0.004) + a success ($0.01).
3. Compare document.spent_usd with document_cost.total_cost_usd.

Before:

     title      | spent_usd | total_cost_usd | calls | input_tokens
----------------+-----------+----------------+-------+--------------
 TSA-495 ticket |  0.000288 |                |       |
 TSA-495 mixed  |  0.017000 |       0.014000 |     2 |         3200

After:

     title      | spent_usd | total_cost_usd | calls | input_tokens
----------------+-----------+----------------+-------+--------------
 TSA-495 ticket |  0.000288 |       0.000288 |     0 |          200
 TSA-495 mixed  |  0.017000 |       0.017000 |     2 |         4200

Notes

• Rate-limited events written before this change have no token counts, so they
  add cost but 0 tokens.
• A document with only such charges shows calls = 0 and avg_latency_ms = null.

[RomanNabukhotniidev]

good work!

[Dekshad]

LGTM!
Tested locally, works as expected.