feat(api): server-side Zod transforms for full e2e type safety with Treaty client

[andrew-bierman]

Follow-up to #2083. Landed in that PR: 100% removal of axios, full Treaty migration, and a single source-of-truth server type graph. What remains is a class of 28 `as unknown as LocalType` casts at the hook boundary in `apps/expo`, all caused by one root issue.

Root cause

Treaty infers Elysia's response types from the Zod schemas. Where schemas reflect Drizzle column types directly (e.g. `createdAt: z.date()`, `price: z.number().nullable()`), the inferred wire type is `Date` / `number | null`. But the JSON actually sent over the wire serializes Dates to ISO strings. The client consumers (UI components, date formatters, comparisons) are written against the wire reality — `string` / `string | null`.

The mismatch: Treaty types the response as `{ createdAt: Date }`, the actual payload is `{ createdAt: "2026-04-14T..." }`, consumers expect `string`. The 28 casts paper over this at the hook boundary.

Scope

Full fix: wrap every datetime / server-specific column in its response schema with `.transform()` so the wire type matches what consumers already consume.

Affected server schemas — every Zod schema used as a route `response` that mirrors a Drizzle row with Date columns:

• `packages/api/src/schemas/packs.ts`
• `packages/api/src/schemas/packTemplates.ts`
• `packages/api/src/schemas/catalog.ts`
• `packages/api/src/schemas/feed.ts`
• `packages/api/src/schemas/trips.ts` (if extracted)
• `packages/api/src/schemas/guides.ts`
• `packages/api/src/schemas/users.ts`
• `packages/api/src/schemas/weather.ts`
• `packages/api/src/schemas/auth.ts`

Typical transform pattern:
```ts
createdAt: z.date().transform((d) => d.toISOString()),
updatedAt: z.date().transform((d) => d.toISOString()),
expiresAt: z.date().nullable().transform((d) => d?.toISOString() ?? null),
```

Affected client files — 24 hook/store files with `as unknown as` casts that should be removed once the server types align:

```
apps/expo/features/ai-packs/hooks/useGeneratedPacks.ts
apps/expo/features/ai/hooks/useReportContent.ts
apps/expo/features/ai/hooks/useReportedContent.ts
apps/expo/features/ai/hooks/useUpdateReportStatus.ts
apps/expo/features/catalog/hooks/useCatalogItems.ts
apps/expo/features/catalog/hooks/useCatalogItemsCategories.ts
apps/expo/features/catalog/hooks/useSimilarItems.ts
apps/expo/features/pack-templates/hooks/useGenerateTemplateFromOnlineContent.ts
apps/expo/features/pack-templates/store/packTemplateItems.ts
apps/expo/features/packs/hooks/useAllPacks.ts
apps/expo/features/packs/hooks/useDuplicatePack.ts
apps/expo/features/packs/hooks/useImageDetection.ts
apps/expo/features/packs/hooks/usePackDetailsFromApi.ts
apps/expo/features/packs/hooks/usePackGapAnalysis.ts
apps/expo/features/packs/hooks/usePackItemDetailsFromApi.ts
apps/expo/features/packs/hooks/useSeasonSuggestions.ts
apps/expo/features/packs/store/packItems.ts
apps/expo/features/packs/utils/uploadImage.ts
apps/expo/features/trail-conditions/hooks/useTrailConditionReports.ts
apps/expo/features/trips/hooks/useAllTrips.ts
apps/expo/features/weather/hooks/useWeatherAlert.ts
apps/expo/features/weather/lib/weatherService.ts
apps/expo/features/wildlife/hooks/useWildlifeIdentification.ts
```

Approach

1. For each response schema, identify every Date field and wrap with `.transform(d => d.toISOString())`.
2. For nullable Dates, wrap with `.nullable().transform(d => d?.toISOString() ?? null)`.
3. Replace each local type (`Pack`, `PackItem`, `CatalogItem`, `Trip`, etc.) with the Treaty-inferred equivalent derived from `apiClient`:
   ```ts
   type PackListResponse = NonNullable<Awaited<ReturnType>['data']>;
   export type Pack = PackListResponse[number];
   ```
4. Remove each `as unknown as X` cast. Fix any consumer errors that surface.
5. Verify:
   • Root `tsc --noEmit` clean
   • No `as unknown as` in `apps/expo/features`
   • No new `biome-ignore` or `@ts-expect-error`

Done when

• All response schemas transform dates to ISO strings at the wire boundary
• Local domain types (`Pack`, `PackItem`, `CatalogItem`, `Trip`, `Post`, `Comment`, `PackTemplate`, `PackTemplateItem`, `TrailConditionReport`) are derived from Treaty
• Zero `as unknown as` casts in `apps/expo/features` and `apps/expo/lib/api`
• `bun run check-types` and `bun run lint` both green
• No new escape hatches (`any`, `@ts-expect-error`, biome-ignore)

Why separate from #2083

#2083 is at ~150 files touched and merges cleanly today. This cleanup touches a different dimension (server schemas + client type consumers, ~60 files) and benefits from isolated review. Should base on #2083's branch so it merges after.

[andrew-bierman]

Scope correction after worktree investigation

A subagent started this in an isolated worktree and stopped before opening a PR — findings below. The issue as written had two wrong premises that a proper fix needs to address.

Premise 1 was wrong: schemas already use z.string().datetime()

Every response schema in packages/api/src/schemas/ already models dates as z.string().datetime(), not z.date(). There are no z.date() fields to wrap with .transform(...). "Phase 1" as originally written is a no-op.

The actual root cause is route-level, not schema-level

No Elysia route under packages/api/src/routes/ declares a response: schema. Without response:, Treaty infers the return type from the handler's return value — which is Drizzle's row shape (real Date objects, nullable columns as string | null). A few routes manually call .toISOString() on specific fields before returning, but most don't. The existing z.string().datetime() schemas are never referenced from any route.

Two viable paths:

• A: wire response: onto every route using the existing Zod schemas; Elysia's @effect/schema-style coercion will serialize the return value to match. Requires auditing that every route's actual return shape satisfies its declared schema (many currently don't).
• B: leave route declarations alone, normalize every return site server-side (.toISOString(), ?? null). Untyped at the route boundary, works at runtime.

Premise 2 was wrong: it's not only about dates

Even with dates handled, the hand-written local types are fundamentally incompatible with Treaty-inferred ones. The nullability convention differs: local string | undefined, server string | null. Flipping local types to | null cascades through ~100+ consumer files (50+ import features/packs/types, 13 import features/catalog/types, dozens more across pack-templates, trail-conditions, feed, guides). Each needs revisiting for ?? fallback or optional-chain handling.

A verified 1-cast probe removal (useAllTrips.ts) surfaced type errors that have nothing to do with dates — purely the null/undefined convention mismatch.

Recommended split

The "one PR, 28 → 0 casts" framing isn't feasible. Realistic breakdown:

• PR A (server): declare response: on every route using the existing z.string().datetime() schemas. Audit the runtime return shape satisfies each declared schema. Fixes ~20-30% of client casts where the shape was only drifting on dates.
• PR B (per-domain): migrate each local type (Trip, Pack, PackItem, CatalogItem, Guide, Post, Comment, PackTemplate, PackTemplateItem, TrailConditionReport) to Treaty-inferred, one domain per PR. Each carries its own consumer fanout fixes. Largest reviewable unit.
• PR C (cleanup): remove the final casts, delete the utility types, verify zero as unknown as in apps/expo/features.

Estimated: 2-3 focused days of work across the three PRs. The subagent worktree was discarded cleanly (no commits).