Add GitHub Actions CI, wire ESLint, lock in Caddy

CI pipeline with parallel jobs for fast feedback:
- typecheck, lint, unit tests, build run in parallel
- e2e tests run after build
- Playwright artifacts uploaded on failure

ESLint wired into both apps with sensible defaults:
- underscore-prefixed unused vars allowed
- root package.json marked as type: module

Architecture updated: Caddy as reverse proxy with auto HTTPS.

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

package.json

@@ -2,6 +2,7 @@
   "name": "trails.cool",
   "private": true,
   "description": "Collaborative route planning + federated activity sharing for outdoor enthusiasts",
+  "type": "module",
   "license": "MIT",
   "packageManager": "pnpm@10.6.5",
   "engines": {