Production cleanup: enable R8, add ProGuard rules, validate API input

- Enable R8 minification and resource shrinking for release builds
- Add ProGuard keep rules for Ktor, kotlinx.serialization, Room
- Validate hour/minute range in POST /set endpoint
- Guard wake lock release on server start failure
- Remove unused template colors from colors.xml
- Rewrite README with curl examples, security note, troubleshooting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/build.gradle.kts

@@ -34,7 +34,8 @@ android {
     }
     buildTypes {
         release {
-            isMinifyEnabled = false
+            isMinifyEnabled = true
+            isShrinkResources = true
             proguardFiles(
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro"