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/activities.ts

@@ -0,0 +1,45 @@
+import { z } from "zod";
+
+/** Activity summary for list views */
+export const ActivitySummarySchema = z.object({
+  id: z.string().uuid(),
+  name: z.string(),
+  description: z.string(),
+  routeId: z.string().uuid().nullable(),
+  routeName: z.string().nullable(),
+  distance: z.number().nullable(),
+  duration: z.number().nullable(),
+  elevationGain: z.number().nullable(),
+  elevationLoss: z.number().nullable(),
+  startedAt: z.string().datetime().nullable(),
+  geojson: z.string().nullable(),
+  createdAt: z.string().datetime(),
+});
+
+/** Full activity detail */
+export const ActivityDetailSchema = ActivitySummarySchema.extend({
+  gpx: z.string().nullable(),
+  photos: z.array(z.string().url()),
+});
+
+/** Paginated activity list response */
+export const ActivityListResponseSchema = z.object({
+  activities: z.array(ActivitySummarySchema),
+  nextCursor: z.string().nullable(),
+});
+
+/** Create activity request */
+export const CreateActivityRequestSchema = z.object({
+  name: z.string().min(1).max(200),
+  description: z.string().max(5000).default(""),
+  gpx: z.string().optional(),
+  routeId: z.string().uuid().optional(),
+  startedAt: z.string().datetime().optional(),
+  duration: z.number().optional(),
+  distance: z.number().optional(),
+});
+
+export type ActivitySummary = z.infer<typeof ActivitySummarySchema>;
+export type ActivityDetail = z.infer<typeof ActivityDetailSchema>;
+export type ActivityListResponse = z.infer<typeof ActivityListResponseSchema>;
+export type CreateActivityRequest = z.infer<typeof CreateActivityRequestSchema>;