feat(budget): source_anomalies and account_category on budget accounts

[makegov-mark]

What

Supports the Tango API v5.7.0 and v5.8.0 changes to /api/budget/accounts/, including data_through_period (the File A fiscal period, 1–12, an account-year's figures run through; null with no File A data), read with models::budget_data_through_period and filtered with data_through_period / _gte / _lte / _isnull:

• source_anomalies (default shape): each code (contract_exceeds_obligations capped; assistance_exceeds_obligations and contract_without_obligations flagged), every element field including likely_cause and source.rows[], a worked example, and a note that codes may grow and that there is no anomaly filter.
• account_category (budgetary / credit_financing): the field, the account_category / account_category__in filters, why credit financing accounts are non-budgetary and left out of organization budget totals, and the advice to filter account_category=budgetary before summing.
• Default ordering: accounts with no enacted_ba sort last within an FY, ties broken by id.
• Both fields carry a neutral note that they may be empty until the next data refresh.

Pages

• docs/api-reference/budget.md: filter row, default shape, ordering, new "Account category" and "Source anomalies" sections.
• docs/data-dictionary/budget.md: account_category row, new "Source anomalies" section.
• docs/guides/how-to/explore-federal-budget.md: taxonomy filter, budget_appropriation note, new "Summing accounts" and "Source-data anomalies" subsections.
• CHANGELOG.md: Unreleased entry.

The element example uses the producer's actual message, affected_fields, dataset and file_c_source values; dollar amounts, PIID and TAS are placeholders.

Relationship to #85

#85 (budget known-issues notice) touches the same three pages. This branch is off main and edits different sections, but whichever lands second may need a small rebase.

Testing

• mkdocs build --clean: builds; only the existing warnings for files fetched at build time.
• scripts/check_doc_links.py --internal-only: all internal links OK.
• scripts/check_api_drift.py --surface against tango origin/main: budget filters and shapes documented; the one coverage error is the unrelated ebuy/requests endpoint (covered by #79).
• markdownlint: no new findings other than the file's existing MD046 fenced-block pattern.

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