Curation v2 step 2: Claude Opus 5 editor, the Brief, per-provider budgets

- AnthropicBackend (Messages API, cached system block, output_config.effort,
  server-side fallbacks, refusal surfaced as an error); Llms { bulk, editor }
  with editor_or_bulk(); PriceTable-based UsageMeter per provider.
- [anthropic], [editorial], deepseek.max_concurrent_requests and
  curation.max_article_count config; startup logs resolved providers.
- Budget day is the UTC date of started_at, preloaded from
  runs.provider_costs_json; finish_run writes provider_costs_json and
  config_json. Stage A batches run concurrently with per-batch budget checks.
- Editor prompt with one-line "why" per pick; no minimum lineup size;
  --max-articles is a ceiling; top-up branch deleted; why stored on picks and
  issue_articles.why and rendered in chapters and In this issue.
- Summaries on the editor client (3k-token input, concurrency 4, bulk then
  excerpt fallback); "The Brief" replaces From the Editor; section intros gone.
- Colophon carries per-provider costs and models.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A1rCLQeKBgnBo3oTgHuTMe

tests/fixtures/claude_brief.json

@@ -0,0 +1,3 @@
+{
+  "brief": "The lead, \"Migrating 40TB off Postgres\", is the rare migration write-up that keeps its failures in: two aborted cutovers, the rollback that took longer than the move, and a bill at the end. Read it first while the coffee is hot; it rewards attention and it is long.\n\nThe local desk answers with \"The MBTA slow-zone dataset\", which finally puts the T's own numbers into a shape a rider can argue with, and the charts do more persuading than a year of press releases. \"A failover story you'd argue with\" rounds out the engineering pages with a Postgres HA design that disagrees with the lead on almost every point, which is exactly why the two belong in the same issue. The issue is shorter than usual because a thin Friday is a good excuse to finish the long one properly rather than skim six."
+}