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>

.gitea/workflows/release.yml

@@ -0,0 +1,99 @@
+# Builds a signed, semantically versioned APK of the PRODUCTION APP on every
+# v* tag and attaches it to a Gitea release. The website resolves "latest"
+# through the Gitea API at request time, so publishing a release is the ONLY
+# step — no site redeploy.
+#
+# PARKED until the production app lands in echolot-app/ (the prober is a
+# one-shot tool and deliberately not CI-built). Runs only on v* tags, so it
+# stays inert until the first tag is pushed. When echolot-app exists it needs
+# the tag-derived version wiring in its build.gradle.kts:
+#   versionName from -PversionName, versionCode from -PversionCode,
+#   keystore.properties (gitignored) for release signing.
+#
+# Required repo secrets (Settings → Actions → Secrets):
+#   KEYSTORE_B64        base64 of the release keystore (base64 -w0 echolot.jks)
+#   KEYSTORE_PASSWORD   store password
+#   KEY_ALIAS           key alias
+#   KEY_PASSWORD        key password
+# RELEASE_TOKEN is NOT needed: the built-in GITHUB_TOKEN can create releases.
+#
+# Tag to release:  git tag v0.2.0 && git push origin v0.2.0
+
+name: release-apk
+on:
+  push:
+    tags: ["v*.*.*"]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: echolot-app
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: "17"
+
+      - name: Install Android SDK cmdline tools
+        run: |
+          sudo mkdir -p /opt/android-sdk/cmdline-tools
+          curl -sSL -o /tmp/clt.zip https://dl.google.com/android/repository/commandlinetools-linux-11076708_latest.zip
+          sudo unzip -q /tmp/clt.zip -d /opt/android-sdk/cmdline-tools
+          sudo mv /opt/android-sdk/cmdline-tools/cmdline-tools /opt/android-sdk/cmdline-tools/latest
+          echo "ANDROID_HOME=/opt/android-sdk" >> "$GITHUB_ENV"
+          yes | sudo /opt/android-sdk/cmdline-tools/latest/bin/sdkmanager --licenses > /dev/null
+          sudo /opt/android-sdk/cmdline-tools/latest/bin/sdkmanager "platforms;android-35" "build-tools;35.0.0" > /dev/null
+
+      - name: Derive version from tag
+        id: ver
+        run: |
+          NAME="${GITHUB_REF_NAME#v}"                      # v1.2.3 -> 1.2.3
+          IFS='.' read -r MAJ MIN PAT <<< "${NAME%%-*}"    # strip -rc1 etc. for the code
+          CODE=$((MAJ * 10000 + MIN * 100 + PAT))
+          echo "name=$NAME" >> "$GITHUB_OUTPUT"
+          echo "code=$CODE" >> "$GITHUB_OUTPUT"
+
+      - name: Materialize keystore
+        env:
+          KEYSTORE_B64: ${{ secrets.KEYSTORE_B64 }}
+        run: |
+          echo "$KEYSTORE_B64" | base64 -d > release.jks
+          cat > keystore.properties <<EOF
+          storeFile=release.jks
+          storePassword=${{ secrets.KEYSTORE_PASSWORD }}
+          keyAlias=${{ secrets.KEY_ALIAS }}
+          keyPassword=${{ secrets.KEY_PASSWORD }}
+          EOF
+
+      - name: Build release APK
+        run: |
+          ./gradlew --no-daemon :app:assembleRelease \
+            -PversionName=${{ steps.ver.outputs.name }} \
+            -PversionCode=${{ steps.ver.outputs.code }}
+
+      - name: Rename + checksum
+        id: apk
+        run: |
+          APK="echolot-prober-${{ steps.ver.outputs.name }}.apk"
+          cp app/build/outputs/apk/release/app-release.apk "$APK"
+          sha256sum "$APK" > "$APK.sha256"
+          echo "file=$APK" >> "$GITHUB_OUTPUT"
+
+      - name: Create Gitea release + attach assets
+        env:
+          TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          API: ${{ github.server_url }}/api/v1/repos/${{ github.repository }}
+        run: |
+          REL=$(curl -sf -X POST "$API/releases" \
+            -H "Authorization: token $TOKEN" -H "Content-Type: application/json" \
+            -d "{\"tag_name\":\"$GITHUB_REF_NAME\",\"name\":\"$GITHUB_REF_NAME\",\"draft\":false,\"prerelease\":false}")
+          ID=$(echo "$REL" | jq -r .id)
+          for f in "${{ steps.apk.outputs.file }}" "${{ steps.apk.outputs.file }}.sha256"; do
+            curl -sf -X POST "$API/releases/$ID/assets?name=$f" \
+              -H "Authorization: token $TOKEN" \
+              -F "attachment=@$f"
+          done