License the project; add root README, website, and parked release CI

- Code: GPL-3.0-or-later (LICENSE, SPDX headers on all .kt/.aidl).
  Specs in docs/: CC-BY-4.0 (docs/LICENSE). Rationale in build-status.md;
  server decided GPL (not AGPL).
- Root README for the public repo.
- web/: Cloudflare Worker site for echo-lot.app. /apk resolves the newest
  APK from the Gitea latest-release API at request time (edge-cached 5 min),
  so tagging a release is the only publish step. /fdroid, /source, and a
  manual DOWNLOAD_URL fallback are wrangler vars.
- .gitea/workflows/release.yml: tag-driven (v*) signed semver APK builds for
  the future production app in echolot-app/. Parked; the prober is
  deliberately not CI-built.
- Fix UserService.kt: drop the explicit secondary constructor that
  conflicted with the implicit primary (never compiled before — first
  local build caught it). Prober now builds: :app:assembleDebug OK.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

echolot-prober/README.md

@@ -57,6 +57,11 @@ The debug APK lands in `app/build/outputs/apk/debug/`.
   ethernet, attach an adapter first.
 - BLE advertising support is genuinely chipset-dependent; a UNSUPPORTED here is a real finding.
 
+## License
+
+GPL-3.0-or-later, like the rest of the Echolot code — see [`../LICENSE`](../LICENSE). The specs in
+[`../docs/`](../docs/) are CC-BY-4.0.
+
 ## Relationship to the specs
 
 Result IDs mirror the `measurement-schema.md` test-type registry where one exists, and the JSON