feat(nzb-postproc): publish typed terminal failure codes

[thedancingdeveloper]

Summary

Ports typed terminal failure codes from MrVampy/rustnzb (e0870d8): persist an engine-owned failure code with each terminal history row instead of making consumers parse human-readable diagnostic prose, and make the durable terminal row authoritative over a job's lingering queue view.

⚠️ Stacked on #149 (idempotent queue admission). This PR's base is feat/idempotent-queue-admission, because it extends QueueAdmissionState::History (added there) and its DB migration follows that one's. Review/merge #149 first; the diff here is the failure-code change alone.

What it adds

• JobFailureCode enum (nzb-core) with stable snake_case wire values (Display + FromStr): articles_unavailable, repair_failed, archive_password_required, archive_invalid, storage_unavailable, download_failed.
• Post-processing assigns codes from causal stage state: run_pipeline threads a failure_code (articles-unavailable, repair-failed) and run_extract_stage returns a typed code — including a typed ArchivePasswordRequired error that replaces the anyhow::bail!("archive is password-protected") prose consumers previously had to string-match (this prose-matching debt was still live on main's unpack.rs).
• Persistence: HistoryEntry.failure_code (DB migration v12 adds the column and backfills existing failed rows to download_failed); history_insert enforces the invariant that a Failed row carries exactly one code and a non-Failed row carries none.
• Authoritative terminal row: queue_manager tracks the code in memory across pause/resume/clear and writes it on terminal transition; queue_admission_observe now consults the history row before the queue view and reports the code via QueueAdmissionState::History. Exposed on the history API response.

Port notes

• DB migration renumbered v10 → v12 (main's v10/v11 are damage_ledger / queue_admissions).
• History SQL column indices adjusted for main's retry_data column (fork didn't have it); new failure_code column appended to SELECTs to avoid index churn.
• Pipeline changes reapplied onto main's run_pipeline_with_cleanup structure (fork's was run_pipeline_with_resources).
• QueueAdmissionState::History extends the variant added in feat(nzb-web): idempotent queue admission via Idempotency-Key #149.
• Fork flake/workspace-version bumps excluded.

Tests

• JobFailureCode wire-value roundtrip; typed ArchivePasswordRequired survives as a downcastable error.
• run_extract_stage / run_pipeline code assignment (ArchiveInvalid, ArticlesUnavailable).
• DB migration backfill (failed→download_failed, completed untouched); history invariant (Failed ⇔ code present); terminal-history authority over the queue view.
• queue_manager terminal-code persistence (ArchiveInvalid for raw-artifacts, DownloadFailed fallback).
• Full nzb-core / nzb-postproc / nzb-web / rustnzb suites pass; cargo clippy --workspace --all-targets + cargo fmt --check clean.

Original author: @MrVampy (credited via Co-Authored-By; ported by hand due to migration collisions and divergent pipeline/history code on main).

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