fix: reuse cached gateway downloads within a 10-minute TTL (#915)

Every tap on a gateway attachment re-downloaded the whole file: the
cache write used a random UUID prefix, so no two fetches ever produced
the same filename and old copies were never looked up.

Cache files are now named deterministically from the gateway path
(UUID3 hash + filename), so repeat opens within the TTL (10 min) reuse
the existing download and skip the network entirely. Stale entries fall
through to a fresh fetch that overwrites the cache via temp-file +
rename, so a failed re-download never destroys a previously-good copy.
Content types are remembered in-memory per path so cache hits still
hand the viewer the right MIME.

Also adds isFreshCacheFile()/cacheFileNameFor() unit tests.

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/test/java/com/m57/hermescontrol/data/remote/GatewayFileClientTest.kt

@@ -13,6 +13,7 @@ import kotlinx.coroutines.test.runCurrent
 import kotlinx.coroutines.test.runTest
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotEquals
 import org.junit.Assert.assertNull
 import org.junit.Assert.assertTrue
 import org.junit.Before
@@ -108,6 +109,31 @@ class GatewayFileClientTest {
         assertEquals("a b.png", GatewayFileClient.parseFilename("inline; filename*=UTF-8''a%20b.png"))
     }
 
+    @Test
+    fun `cacheFileNameFor is deterministic per path`() {
+        val first = GatewayFileClient.cacheFileNameFor("/tmp/report.pdf")
+        val second = GatewayFileClient.cacheFileNameFor("/tmp/report.pdf")
+        val other = GatewayFileClient.cacheFileNameFor("/tmp/other.pdf")
+        assertEquals(first, second)
+        assertNotEquals(first, other)
+        assertTrue(first.endsWith("-report.pdf"))
+    }
+
+    @Test
+    fun `isFreshCacheFile respects the TTL`() {
+        val dir = java.io.File(System.getProperty("java.io.tmpdir"), "gwcache_${System.nanoTime()}").apply { mkdirs() }
+        try {
+            val fresh = java.io.File(dir, "fresh").apply { writeText("x") }
+            fresh.setLastModified(System.currentTimeMillis() - 60_000L) // 1 min old
+            val stale = java.io.File(dir, "stale").apply { writeText("x") }
+            stale.setLastModified(System.currentTimeMillis() - 60 * 60 * 1000L) // 1 h old
+            assertTrue(GatewayFileClient.isFreshCacheFile(fresh))
+            assertFalse(GatewayFileClient.isFreshCacheFile(stale))
+        } finally {
+            dir.deleteRecursively()
+        }
+    }
+
     @Test
     fun `copyChunked copies input to output byte-for-byte`() =
         runTest {