[Archive] Make external-import previews handle archived dedupe matches

[Chris0Jeky]

Confirmed introduced MEDIUM from #2932 comment 3982295771. ExternalImportService includes archived cards in provider/profile/dedupe matching. When incoming data changes, preview reports an update; applying calls Card.Update, which rejects the archived card and rolls back the entire import with an explicit domain error. Unchanged matching rows are skipped. No partial write or data loss occurs.

Residual: classify archived matches during planning so preview describes the actual permitted outcome, using an explicit conflict or another documented non-mutating disposition. Do not silently restore or duplicate the archived card. Test a mixed batch with one changed archived match, an unchanged archived match, ordinary active upserts, and preview/apply parity.

Tracked under the bounded review rule; no active claim. #2932 (comment)

[Chris0Jeky]

[Codex lane claim v2]
lane: alpha-product-trust
issue: #2935
base: 54e4c0a
owned paths: backend/src/Taskdeck.Application/Services/ExternalImportService.cs; new focused ExternalImportArchivedMatchTests; a dedicated docs/analysis evidence note
shared-path leases: none; STATUS/MASTERPLAN and the active relation/estimate/archive-editor stack are excluded
parallel-safe work: no edits to card lifecycle/domain, proposal vocabulary, board JSON portability, migrations, UI, or CI
status: active

Implement a non-mutating conflict for changed archived dedupe matches during planning, preserve unchanged archived skips, and prove whole-batch refusal plus active-card control behavior in focused tests. Existing open PRs and this issue's comments checked; #3066's changed-file list does not overlap these paths. This environment currently lacks dotnet; executable .NET qualification will be disclosed, not inferred. PR only, no merge or deployment.

[Chris0Jeky]

[Codex lane release v2]
lane: alpha-product-trust
issue: #2935
PR: #3068 (draft)
exact head: 56d7a8e
result: implementation submitted; .NET qualification outstanding; not merged
released paths: ExternalImportService.cs, new ExternalImportArchivedMatchTests.cs, docs/analysis/2026-09-12-archived-import-conflicts.md
status: no active writer from this session

Added an explicit ArchivedExistingMatch planning conflict for title/description/move changes, after the unchanged-row skip and before planned upserts. Existing whole-batch conflict refusal ensures no transaction or partial writes. Archived/active duplicate ambiguity is preserved. Supplied 13 cases, including a real CSV-adapter preview/Apply comparison and mixed-batch/no-write controls.

These cases have NOT been compiled or executed locally: dotnet is absent and the attempted command failed to start. The draft records the exact command and required next validation. Docs/governance/link/whitespace checks pass; remote source/test hashes match the local files. Hosted checks started, but no complete verdict or independent review is claimed.

CLAUDE_SYNC_PACKET
canonical truth changes: none; PR candidate only
issue/project changes: issue remains open; draft PR carries Priority III/backend/bug; no project-wide sync
architecture impact: no lifecycle, authorization, migration, metadata or CI-policy change
next action: run the exact-head ExternalImport test filter, required backend/API gates and independent review before promoting the draft
human/device/screen-reader/provider/release/hosting gates: all unchanged