Add @trails-cool/api package with Zod schemas for REST API contract

Single source of truth for the Journal REST API:
- API_VERSION semver constant (1.0.0)
- Endpoint path constants (routes, activities, auth, uploads, push)
- Zod schemas for all request/response shapes:
  - Routes: list, detail, create, update, compute
  - Activities: list, detail, create
  - Auth: token exchange, refresh, device management
  - Discovery: instance info + API version
  - Uploads: presigned URL flow
  - Errors: structured error response with field-level details
- Cursor-based pagination schemas
- TypeScript types inferred from Zod (z.infer)

Used by Journal server (validation) and mobile client (type safety).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/api/src/index.ts

@@ -0,0 +1,56 @@
+export { API_VERSION } from "./version.ts";
+export { ENDPOINTS } from "./endpoints.ts";
+
+// Error schemas
+export {
+  ApiErrorResponseSchema, FieldErrorSchema, ERROR_CODES,
+  type ApiErrorResponse, type FieldError,
+} from "./errors.ts";
+
+// Pagination
+export {
+  PaginationQuerySchema, PaginatedResponseSchema,
+  type PaginationQuery, type PaginatedResponse,
+} from "./pagination.ts";
+
+// Discovery
+export {
+  DiscoveryResponseSchema,
+  type DiscoveryResponse,
+} from "./discovery.ts";
+
+// Auth
+export {
+  TokenExchangeRequestSchema, TokenResponseSchema,
+  DeviceSchema, DeviceListResponseSchema,
+  type TokenExchangeRequest, type TokenResponse,
+  type Device, type DeviceListResponse,
+} from "./auth.ts";
+
+// Routes
+export {
+  RouteSummarySchema, RouteDetailSchema, RouteVersionSchema,
+  RouteListResponseSchema,
+  CreateRouteRequestSchema, UpdateRouteRequestSchema,
+  ComputeRouteRequestSchema,
+  type RouteSummary, type RouteDetail, type RouteVersion,
+  type RouteListResponse,
+  type CreateRouteRequest, type UpdateRouteRequest,
+  type ComputeRouteRequest,
+} from "./routes.ts";
+
+// Activities
+export {
+  ActivitySummarySchema, ActivityDetailSchema,
+  ActivityListResponseSchema,
+  CreateActivityRequestSchema,
+  type ActivitySummary, type ActivityDetail,
+  type ActivityListResponse,
+  type CreateActivityRequest,
+} from "./activities.ts";
+
+// Uploads
+export {
+  PresignedUploadRequestSchema, PresignedUploadResponseSchema,
+  type PresignedUploadRequest, type PresignedUploadResponse,
+} from "./uploads.ts";