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/package.json

@@ -0,0 +1,10 @@
+{
+  "name": "@trails-cool/api",
+  "private": true,
+  "version": "0.0.1",
+  "type": "module",
+  "main": "src/index.ts",
+  "dependencies": {
+    "zod": "^3.25.0"
+  }
+}