refactor(backend): split ThumbnailService.generate into stages with distinct logs

Addresses @felixbrandt — fix(backend): "the two try blocks in generate()
overlap — a save failure logs 'generation failed' even though the
thumbnail is already in S3 as an orphan".

generate() now orchestrates four stages, each in its own try+log:
  readSourceImage / encodeThumbnail / uploadToStorage / persistThumbnailMetadata

persistThumbnailMetadata emits the distinct "orphaned in storage as <key>"
log line so an operator can see database-side failures after the upload
completed. The deterministic key ensures the next run overwrites cleanly,
so the orphan is self-healing.

Also extracts THUMBNAIL_KEY_PREFIX/SUFFIX constants with a comment
explaining the deterministic-overwrite contract.

Adds test: generate_returnsFailed_whenPersistThrows_butUploadSucceeded.

Refs #307

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/test/java/org/raddatz/familienarchiv/service/ThumbnailServiceTest.java

@@ -167,6 +167,24 @@ class ThumbnailServiceTest {
         verify(documentRepository, never()).save(any());
     }
 
+    @Test
+    void generate_returnsFailed_whenPersistThrows_butUploadSucceeded() throws IOException {
+        // Covers the "orphan thumbnail" edge case: S3 upload succeeded but the
+        // entity update blew up. We must still return FAILED so the backfill
+        // tally is honest, without losing the fact that we already put bytes in S3.
+        Document doc = makeDoc("application/pdf", "documents/letter.pdf");
+        when(fileService.downloadFileStream(anyString()))
+                .thenReturn(new ByteArrayInputStream(createSamplePdf()));
+        when(documentRepository.save(any()))
+                .thenThrow(new RuntimeException("constraint violation"));
+
+        ThumbnailService.Outcome outcome = thumbnailService.generate(doc);
+
+        assertThat(outcome).isEqualTo(ThumbnailService.Outcome.FAILED);
+        verify(s3Client).putObject(any(PutObjectRequest.class), any(RequestBody.class));
+        verify(documentRepository).save(any());
+    }
+
     // ─── helpers ──────────────────────────────────────────────────────────────
 
     private Document makeDoc(String contentType, String filePath) {