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>

web/wrangler.jsonc

@@ -0,0 +1,35 @@
+{
+  "$schema": "node_modules/wrangler/config-schema.json",
+  "name": "echolot-site",
+  "main": "src/index.js",
+  "compatibility_date": "2026-07-30",
+
+  // Static files are served directly from the edge; the Worker only runs for
+  // paths that do not match a file (i.e. /apk and /fdroid below).
+  "assets": {
+    "directory": "./public",
+    "binding": "ASSETS"
+  },
+
+  // Both hostnames must exist as zones/records in the same Cloudflare account.
+  "routes": [
+    { "pattern": "echo-lot.app", "custom_domain": true },
+    { "pattern": "www.echo-lot.app", "custom_domain": true }
+  ],
+
+  // Nothing here needs touching per release: /apk asks the Gitea API for the
+  // latest release at request time (edge-cached 5 min). Any empty value makes
+  // its route fall back to the homepage's install section, so nothing 404s
+  // before the first release exists.
+  "vars": {
+    // Gitea repo API base, e.g. "https://git.example.net/api/v1/repos/mram/echolot".
+    // The repo (or at least its releases) must be publicly readable.
+    "GITEA_REPO_API": "",
+    // Manual override / fallback while GITEA_REPO_API is unset or unreachable.
+    "DOWNLOAD_URL": "",
+    // F-Droid listing, once it exists: https://f-droid.org/packages/app.echo_lot.app/
+    "FDROID_URL": "",
+    // Public source URL, the footer + /source target.
+    "SOURCE_URL": ""
+  }
+}