feat: Implement caching and rate limiting for API requests

android/app/src/main/kotlin/org/xtimms/psu/schedule/ScheduleApp.kt

@@ -22,7 +22,7 @@ class ScheduleApp : Application() {
     override fun onCreate() {
         super.onCreate()
         settings = Settings(this)
-        repository = Repository(this, Api(BuildConfig.API_URL), settings)
+        repository = Repository(this, Api(BuildConfig.API_URL, cacheDir), settings)
         notes = NotesStore(this)
         attendance = AttendanceStore(this)
         Reminders.createChannel(this)

android/app/src/main/kotlin/org/xtimms/psu/schedule/data/Api.kt

@@ -6,10 +6,12 @@ import kotlinx.coroutines.withContext
 import kotlinx.serialization.json.Json
 import kotlinx.serialization.json.jsonObject
 import kotlinx.serialization.json.jsonPrimitive
+import okhttp3.Cache
 import okhttp3.HttpUrl
 import okhttp3.HttpUrl.Companion.toHttpUrl
 import okhttp3.OkHttpClient
 import okhttp3.Request
+import java.io.File
 import java.io.IOException
 import java.util.concurrent.TimeUnit
 
@@ -22,9 +24,12 @@ val json = Json {
 }
 
 /** The schedule server's HTTP API. Every error message is ready to show. */
-class Api(baseUrl: String) {
+class Api(baseUrl: String, cacheDir: File? = null) {
     private val base: HttpUrl = baseUrl.toHttpUrl()
     private val client = OkHttpClient.Builder()
+        // The server marks answers "no-cache" with an ETag: OkHttp keeps them here and asks
+        // again with If-None-Match, so an unchanged semester comes back as an empty 304.
+        .apply { if (cacheDir != null) cache(Cache(File(cacheDir, "http"), 10L * 1024 * 1024)) }
         .connectTimeout(10, TimeUnit.SECONDS)
         // The first request after a server start can wait for the catalog index.
         .readTimeout(60, TimeUnit.SECONDS)

android/app/src/main/kotlin/org/xtimms/psu/schedule/data/Models.kt

@@ -17,7 +17,6 @@ data class Timetable(
     val days: List<Day> = emptyList(),
     val weekly: List<WeeklyDay> = emptyList(),
     val notes: List<String> = emptyList(),
-    val generatedAt: String? = null,
 )
 
 @Serializable