Phase 4: convert live pipeline to Result chains (functional refactor plan)

[jaybarden1]

Summary

Phase 4 of docs/wallpaper-scrapper-functional-refactor-plan.md — the live scrape pipeline now flows through Result<T, ScrapeError> end to end.

Pipeline conversion

• Pages (SearchResultsPage, SubscriptionsImagesListPage, TopWallpapersPage, ImagePage): every public method returns Result<T, ScrapeError>; Playwright failures wrapped once via Try → PageLoadFailed; try/catch-log-rethrow shells removed (logging moves to LogFailure at workflow level); page ??= lazy init → EnsurePageAsync() per class
• ImagePageOutcome DU (ScrapedImage | SkippedImage, both carrying RawTags) replaces ImagePageResult; scraped-tag persistence moved to ImagePageService — exactly one save per page (permanently fixes the historical B2 double-save); ImagePage drops IScrapedTagRepository
• ImagePageService.ProcessImagePageAsync → Result chain (outcome → download → save → broadcast → persist → classify); retry-once expressed via new RetryOnceAsync around IImageRetriever only; final failure surfaces as ImageDownloadFailed Result (retires the Phase 3 InvalidOperationException deviation); SkiaSharp probe extracted to injectable IImageDimensionReader
• ConfigurationSaver → Result<Unit, ScrapeError> + pure CategoryMerger.MergeCategories
• FileClassificationService → Result<Unit, ScrapeError> + pure ClassificationMatcher; stray early SaveChangesAsync removed (single trailing save); EnsureTracked stops EF re-inserting existing categories reached via navigation
• ImportExportService → ScrapeError errors + Validation<T> DTO validation reporting ALL invalid rows at once (ImportFailed/ValidationFailed cases)
• SearchWorkflow.RunAsync(ct) → Result<Unit, ScrapeError>, single LogFailure, redundant logger param dropped

FunctionalParadigm

• RetryExtensions.RetryOnceAsync (pinned: 1 attempt on success, fail-then-success = 2 attempts + 1 retry callback, fail-fail = second failure returned)
• Result<T,E> and Exceptional<T> hierarchies closed (private protected ctor): Match is now genuinely exhaustive and test doubles can no longer manufacture foreign subtypes — root-cause fix that kept the "no pattern-matching on Result" style rule intact

Scope boundaries (per plan)

• Subscriptions/TopWallpapers workflows minimally adapted; failure propagation from GetTheImagePagesAsync restored + pinned (full conversion is Phase 5)
• Views/MainWindow bridge via ToStringError (full composition rework is Phase 6)
• Also includes the SqliteConnectionStringProvider config-key change (ScrapeConfiguration:ConnectionStrings:Sqlite) and its test alignment

Review

c-sharp-reviewer: 2 blockers (missing IImageDimensionReader DI registration; swallowed Results in Subscriptions/TopWallpapers loops), 4 should-fix, 3 nits — all fixed and re-verified. Repeat-offence check vs Phases 0–3: 3 repeats found (2× undeclared/unpinned deviation, 1× factory skip), all corrected; guard-clause and consistency-sweep classes were clean this phase.

Test plan

• TDD: red tests committed first (246ca57), implementation + review fixes after
• Full suite green: 347 Wallpaper.Scrapper (344 contract + 3 review-pinning) + 127 FunctionalParadigm + 92 Utilities (1 pre-existing skip); build clean under TreatWarningsAsErrors

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