Fix ithmb append mode: preserve existing data across transfer sessions

- Change ithmb file open from 'wb' (truncate) to 'ab' (append)
- Compute write offset from actual file position via seek(0,2)
- Prevents losing all previous artwork data on each transfer

src/artwork/writer.py

@@ -116,18 +116,24 @@ def write_artworkdb(
         state["index"] += 1
         state["offset"] = 0
         filename = _ithmb_filename(fmt_id, state["index"])
-        ithmb_files[fmt_id] = open(os.path.join(artwork_dir, filename), "wb")
+        ithmb_files[fmt_id] = open(os.path.join(artwork_dir, filename), "ab")
 
     def _write_payload(fmt_id: int, data: bytes) -> IthmbLocation:
         state = ithmb_state[fmt_id]
         if fmt_id not in ithmb_files:
             _open_next(fmt_id)
-        elif state["offset"] > 0 and state["offset"] + len(data) > ITHMB_MAX_SIZE_BYTES:
+        else:
+            f = ithmb_files[fmt_id]
+            f.seek(0, 2)
+            cur_size = f.tell()
+            if cur_size > 0 and cur_size + len(data) > ITHMB_MAX_SIZE_BYTES:
                 _open_next(fmt_id)
+        f = ithmb_files[fmt_id]
+        f.seek(0, 2)
+        offset = f.tell()
+        f.write(data)
+        state["offset"] = offset + len(data)
         filename = _ithmb_filename(fmt_id, state["index"])
-        offset = state["offset"]
-        ithmb_files[fmt_id].write(data)
-        state["offset"] += len(data)
         return IthmbLocation(filename, offset)
 
     try: