Add interruption/atomicity, SFTP, and scheduling tests

- Interruption: failed mid-write leaves original intact (no truncation, no temp
  leftover); a sync that drops after N files resumes cleanly on the next sync
  with all content byte-intact (real network-drop simulation).
- SFTP: live round-trip test against an SFTP server (connect/upload-atomic/
  list/download/overwrite/special-name/delete); skips if endpoint unreachable.
- Scheduling: WorkManager request builders map Wi-Fi-only -> UNMETERED,
  charging-only -> requiresCharging, interval, input data, and tags correctly.

app/src/androidTest/kotlin/com/syncflow/SchedulingTest.kt

@@ -0,0 +1,47 @@
+package com.syncflow
+
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import androidx.work.NetworkType
+import com.syncflow.worker.SyncWorker
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+import java.util.concurrent.TimeUnit
+
+/**
+ * Scheduling/constraint mapping for WorkManager-backed syncs. Verifies the request builders
+ * translate pair settings into the right constraints (Wi-Fi-only, charging-only), interval, input
+ * data, and tags — the deterministic part of scheduling (without waiting for the OS to fire it).
+ */
+@RunWith(AndroidJUnit4::class)
+class SchedulingTest {
+
+    @Test fun periodic_wifiOnly_chargingOnly_intervalAndData() {
+        val req = SyncWorker.buildPeriodicRequest(pairId = 42L, intervalMinutes = 30, wifiOnly = true, chargingOnly = true)
+        val ws = req.workSpec
+        assertEquals(NetworkType.UNMETERED, ws.constraints.requiredNetworkType)
+        assertTrue("charging constraint", ws.constraints.requiresCharging())
+        assertEquals(TimeUnit.MINUTES.toMillis(30), ws.intervalDuration)
+        assertEquals(42L, ws.input.getLong(SyncWorker.KEY_PAIR_ID, -1))
+        assertTrue("sync_42" in req.tags)
+    }
+
+    @Test fun periodic_anyNetwork_noCharging() {
+        val req = SyncWorker.buildPeriodicRequest(pairId = 7L, intervalMinutes = 60, wifiOnly = false, chargingOnly = false)
+        val c = req.workSpec.constraints
+        assertEquals(NetworkType.CONNECTED, c.requiredNetworkType)
+        assertFalse(c.requiresCharging())
+    }
+
+    @Test fun oneTime_constraintsDataAndTag() {
+        val req = SyncWorker.buildOneTimeRequest(pairId = 9L, wifiOnly = true, chargingOnly = false, silent = true)
+        val ws = req.workSpec
+        assertEquals(NetworkType.UNMETERED, ws.constraints.requiredNetworkType)
+        assertFalse(ws.constraints.requiresCharging())
+        assertEquals(9L, ws.input.getLong(SyncWorker.KEY_PAIR_ID, -1))
+        assertTrue(ws.input.getBoolean(SyncWorker.KEY_SILENT, false))
+        assertTrue("sync_9" in req.tags)
+    }
+}