## Why

The monorepo needs a single source of truth for business schema (database tables, validation rules, and shared types) that both the API and web apps can import. Currently `packages/schema` is an empty placeholder with no package.json or source files, forcing each app to duplicate type definitions and validation logic.

## What Changes

- Populate `packages/schema` with a private npm package (`@workspace/schema`) including `package.json`, `tsconfig.json`, and source entry points
- Add Drizzle ORM schema definitions for PostgreSQL (tables, relations, enums)
- Add Superstruct-based runtime validators for request/response shapes
- Export shared TypeScript types derived from both Drizzle and Superstruct definitions
- Add `packages/schema` as a workspace dependency in both `apps/api` and `apps/web`
- Update Turborepo pipeline so `schema` build runs before dependent apps

## Capabilities

### New Capabilities

- `database-schema`: Drizzle ORM table definitions, relations, and enums targeting PostgreSQL
- `validation`: Superstruct schema definitions for runtime validation of data shapes
- `shared-types`: TypeScript type exports re-exported from Drizzle and Superstruct schemas

### Modified Capabilities

None — this is the first real content in `packages/schema`.

## Impact

- `packages/schema` will get a `package.json` and build output; its contents become importable by other workspace packages
- `apps/api` and `apps/web` gain a new workspace dependency
- `turbo.json` needs no changes — `^build` already handles topological build order
- No existing code is removed or modified; this is purely additive