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/endpoints.ts

@@ -0,0 +1,35 @@
+/** Typed API endpoint path constants */
+export const ENDPOINTS = {
+  discovery: "/.well-known/trails-cool",
+
+  auth: {
+    token: "/api/v1/auth/token",
+    devices: "/api/v1/auth/devices",
+    device: (id: string) => `/api/v1/auth/devices/${id}` as const,
+  },
+
+  routes: {
+    list: "/api/v1/routes",
+    create: "/api/v1/routes",
+    detail: (id: string) => `/api/v1/routes/${id}` as const,
+    update: (id: string) => `/api/v1/routes/${id}` as const,
+    delete: (id: string) => `/api/v1/routes/${id}` as const,
+    compute: "/api/v1/routes/compute",
+  },
+
+  activities: {
+    list: "/api/v1/activities",
+    create: "/api/v1/activities",
+    detail: (id: string) => `/api/v1/activities/${id}` as const,
+    delete: (id: string) => `/api/v1/activities/${id}` as const,
+  },
+
+  uploads: {
+    presign: "/api/v1/uploads",
+  },
+
+  push: {
+    subscribe: "/api/v1/push/subscribe",
+    unsubscribe: "/api/v1/push/unsubscribe",
+  },
+} as const;