refactor: safe Tier 0+1 — backend partial-class splits + util tests

[mrviduus]

Summary

Cosmetic refactor pass with zero behaviour change. Three god-class backend files split across C# partial files (compile-identical IL — integration tests + EF model snapshot verify), plus net-new unit-test coverage on critical web + mobile pure utilities. The 1559-LOC VocabularyEndpoints.cs was the largest non-migration backend file pre-PR; now down to 875 LOC. (New largest is ReadingTrackingEndpoints.cs at 920 LOC — not in scope for this PR.)

This is the safe portion of the broader refactor plan (Tier 0 tests + Tier 1 cosmetic splits). Higher-risk items (god-component splits in web, mobile reader factory hook, AI-driven naming) are deferred to post-Play-Store-launch with telemetry data.

Changes

Backend cosmetic splits (zero behaviour change — partial class compiles identically)

File	Before	After	Partials created
AppDbContext.cs	655	104	7 (Catalog/User/Reading/UserBooks/Vocabulary/Ops/Seo/Collections)
VocabularyEndpoints.cs	1559	875	6 (Stats/Settings/Pending/Lookups/Clusters/Admin)
AdminService.cs	977	131	4 (Upload/Editions/Chapters/UserUploads)

Each partial is under 400 LOC. Single domain per file. Shared helpers (e.g. TryGetAuth, EnqueueSsgSafe, MapVocabularyEndpoints) stay in the main file.

Parity verified:

• AppDbContext: all 50 entity configurations present pre + post split
• VocabularyEndpoints: all 32 methods (handlers + helpers) present, 25 routes unchanged in MapVocabularyEndpoints
• AdminService: all 27 methods present
• dotnet ef migrations has-pending-model-changes → "No changes have been made to the model since the last migration" — EF model snapshot byte-identical, no migration churn

Tests added (Tier 0 — additive, no behaviour change)

Web (existing Vitest infra):

• analytics.test.ts — 20 cases / 30 assertions: GA4 event shape contract, gtag fallback, PII boundaries, all 10 wrappers
• dataEvents.test.ts — 10 cases / 13 assertions: CustomEvent bus, useDataChange hook, multi-entity, unmount-leak
• errorUtils.test.ts — 5 cases / 12 assertions: HttpError 404 detection, defensive boundary
• formatTime.test.ts — 8 cases / 15 assertions: hour/minute formatting, edge cases

Mobile (NEW Vitest infra):

• vitest.config.ts — Node env, src/lib/**/*.test.ts include, AsyncStorage alias to in-process mock
• __mocks__/async-storage.ts — in-memory map mirroring the RN module's API surface with __reset() for clean state
• searchUtils.test.ts — 18 cases / 24 assertions: normalization, query matching, documents surprising Cyrillic й → и NFD stripping as intended cross-script search
• features.test.ts — 9 cases / 11 assertions: reader-overlay v2 killswitch cascade
• vocabStatsCache.test.ts — 11 cases / 15 assertions: round-trip, TTL, corrupt-JSON defense, schema-drift rejection

Tests

• Unit (backend): 216 + 313 + 20 = 549 passed (unchanged — splits are IL-identical)
• Unit (web): 474 passed (40 net new)
• Unit (mobile): 38 passed (new — mobile had 0 unit tests pre-PR)
• tsc: clean on web + mobile + backend
• Build: dotnet build textstack.sln → 0 warnings, 0 errors
• EF model: dotnet ef migrations has-pending-model-changes confirms no schema drift

Rollback plan

Single revert — git revert <merge-sha>. No DB migrations, no API changes, no behaviour changes. New test files are additive (revert removes them; no consumer code depends on them).

Self-review findings

• pnpm-lock.yaml accidentally committed (ce944cc) — initially used pnpm add for mobile but mobile uses npm (has package-lock.json tracked). Lock files would have drifted on CI / fresh installs. Caught + fixed in self-review pass: removed pnpm-lock.yaml, ran npm install to update package-lock.json with the new vitest deps. Mobile tests still pass.
• Over-inclusive usings in AdminService partials: all 4 partials import the full superset of 13 usings even though Chapters/UserUploads only need 3-4 each. C# compiler doesn't flag unused usings (no IDE0005 analyzer enabled). Acceptable trade-off — easier to add code without re-validating usings per file. Not worth churn to fix.

Notes

• Why partial classes, not IEntityTypeConfiguration<T> extraction for EF: chose partial class AppDbContext over per-entity config classes to minimise diff risk and keep the existing inline-config pattern (used everywhere in the codebase). EF model snapshot is byte-identical to pre-split — no migration churn. Per-entity config extraction is a separate refactor if the pattern proves useful.
• Why public static partial class for endpoints: ASP.NET Core's WebApplication.MapPost/Get/... works fine with handlers declared across partials of a static class. The only constraint is MapVocabularyEndpoints (which registers routes) and the static helpers (TryGetAuth, ToDto, …) must be visible to all handler partials — which is automatic.
• Mobile Vitest scope is intentionally narrow: only src/lib/**/*.test.ts. Hooks and components are NOT in scope — they need a full @testing-library/react-native + native-module mock setup that's a much bigger infra slice. When a real regression demands it, add it then.
• searchUtils test caught real behavior: й decomposes to и + U+0306 (combining breve) — the NFD strip turns "й" → "и". This means a user typing достоевскии matches Достоевский entries (friendlier for typos than strict NFC). Documented in test as intended.
• No deploy required for the partial-class splits — they're compile-time only. Tests files are additive; CI will pick them up automatically.
• Higher-risk refactors NOT in this PR (deferred to post-Play-Store-launch with telemetry data): LibraryPage.tsx/Search.tsx/ReaderPage.tsx component splits, mobile reader factory hook (premature until 3rd consumer), IEntityTypeConfiguration extraction. Plus blanket NO-GO on offlineDb.ts (903 LOC, IndexedDB schema risk), readerHtml.ts (embedded JS in string), and migrations.

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