feat(frontend): auto-generate TypeScript types from backend OpenAPI schema

Replace manually maintained types in src/lib/types.ts with auto-generated
types from FastAPI's OpenAPI schema using @hey-api/openapi-ts. The bridge
file re-exports generated types with renames, Require<> augmentations for
fields that are optional in the schema but always present in responses, and
manually added relationship fields excluded from OpenAPI.

- Add openapi-ts.config.ts and generate:api npm script
- Generate types into src/lib/api/generated/types.gen.ts
- Rewrite src/lib/types.ts as bridge with re-exports and augmentations
- Fix null vs undefined mismatches in consumer components
- Remove unused manual type definitions from api.ts
- Update AGENTS.md docs with type generation workflow

frontend/AGENTS.md

@@ -19,7 +19,8 @@ frontend/src/
 │   └── profile/         # User profile
 └── lib/
     ├── api.ts           # Typed fetch wrappers (server: PUBLIC_API_BASE, browser: /api)
-    ├── types.ts         # TypeScript types mirroring backend models (manual sync)
+    ├── types.ts         # Type bridge — re-exports from generated OpenAPI types with augmentations
+    ├── api/generated/   # Auto-generated types from backend OpenAPI schema — DO NOT EDIT
     ├── utils.ts         # cn() class merger, bits-ui types
     ├── utils/           # forms.ts (preventIfNotConfirmed), skin-display.ts (label helpers)
     ├── paraglide/       # Generated i18n runtime — DO NOT EDIT
@@ -123,6 +124,7 @@ pnpm check        # Type check + Svelte validation
 pnpm lint         # ESLint
 pnpm format       # Prettier
 pnpm build        # Production build → build/
+pnpm generate:api # Regenerate TypeScript types from backend OpenAPI schema
 ```
 
 ## Anti-Patterns
@@ -130,3 +132,29 @@ pnpm build        # Production build → build/
 - No frontend tests exist. Only linting + type checking.
 - ESLint `svelte/no-navigation-without-resolve` has `ignoreGoto: true` workaround (upstream bug sveltejs/eslint-plugin-svelte#1327).
 - `src/paraglide/` is a legacy output path — active i18n output is in `src/lib/paraglide/`.
+
+## Type Generation
+
+TypeScript types are auto-generated from the FastAPI backend's OpenAPI schema using `@hey-api/openapi-ts`.
+
+### Workflow
+
+1. Generate `openapi.json` from backend: `cd backend && uv run python -c "import json; from main import app; print(json.dumps(app.openapi(), indent=2))" > ../frontend/openapi.json`
+2. Generate types: `cd frontend && pnpm generate:api`
+3. Output lands in `src/lib/api/generated/types.gen.ts` — **never edit this file directly**.
+
+### Architecture
+
+- **Generated types**: `src/lib/api/generated/types.gen.ts` — raw OpenAPI types, auto-generated.
+- **Bridge file**: `src/lib/types.ts` — re-exports from generated types with:
+  - **Renames**: `ProductWithInventory` → `Product`, `ProductListItem` → `ProductSummary`, `UserProfilePublic` → `UserProfile`, `SkinConditionSnapshotPublic` → `SkinConditionSnapshot`.
+  - **`Require<T, K>` augmentations**: Fields with `default_factory` in SQLModel are optional in OpenAPI but always present in API responses (e.g. `id`, `created_at`, `updated_at`, `targets`, `inventory`).
+  - **Relationship fields**: SQLModel `Relationship()` fields are excluded from OpenAPI schema. Added manually: `MedicationEntry.usage_history`, `Routine.steps`, `RoutineStep.product`, `ProductInventory.product`, `Product.inventory` (with augmented `ProductInventory`).
+  - **Manual types**: `PriceTierSource`, `ShoppingPriority` — inline literals in backend, not named in OpenAPI.
+- **Canonical import**: Always `import type { ... } from '$lib/types'` — never import from `$lib/api/generated` directly.
+
+### When to regenerate
+
+- After adding/modifying backend models or response schemas.
+- After adding/modifying API endpoints that change the OpenAPI spec.
+- After updating the bridge file, run `pnpm check` to verify type compatibility.