fix(downloads): probe codecs from ffprobe JSON, not its flat output

ffprobe prints codec_name BEFORE codec_type, so the line parser attributed every
name to the previous stream: a plain H.264+AAC mp4 probed as (aac, None) and the
browser-playability check re-encoded a perfectly good file on EVERY download.
Verified on localdev — the same source now stores without a re-encode pass.

backend/tests/test_download_output.py

@@ -5,7 +5,7 @@ so a missing filepath (an arbitrary-URL extractor) skipped the staging fallback
 with "PosixPath('.') has an empty name" instead of downloading."""
 import pytest
 
-from app.downloads.edit import _concat_quote
+from app.downloads.edit import _concat_quote, parse_probe_streams
 from app.worker import _media_info, _produced_file
 from pathlib import Path, PurePosixPath
 
@@ -62,6 +62,35 @@ class TestMediaInfo:
         assert _media_info(info)["id"] == "vid"
 
 
+class TestParseProbeStreams:
+    # ffprobe emits codec_name BEFORE codec_type, so the old line parser shifted every name onto
+    # the previous stream and an H.264+AAC mp4 came back as (aac, None) → a needless re-encode.
+    def test_reads_each_stream_by_its_own_type(self):
+        payload = '{"streams":[{"codec_name":"h264","codec_type":"video"},{"codec_name":"aac","codec_type":"audio"}]}'
+        assert parse_probe_streams(payload) == ("h264", "aac")
+
+    def test_audio_first_ordering_is_handled(self):
+        payload = '{"streams":[{"codec_name":"AAC","codec_type":"audio"},{"codec_name":"H264","codec_type":"video"}]}'
+        assert parse_probe_streams(payload) == ("h264", "aac")
+
+    def test_extra_streams_do_not_win_over_the_first_of_their_kind(self):
+        payload = (
+            '{"streams":[{"codec_name":"h264","codec_type":"video"},'
+            '{"codec_name":"vp9","codec_type":"video"},'
+            '{"codec_name":"mov_text","codec_type":"subtitle"},'
+            '{"codec_name":"opus","codec_type":"audio"}]}'
+        )
+        assert parse_probe_streams(payload) == ("h264", "opus")
+
+    def test_audio_only_and_video_only(self):
+        assert parse_probe_streams('{"streams":[{"codec_name":"opus","codec_type":"audio"}]}') == (None, "opus")
+        assert parse_probe_streams('{"streams":[{"codec_name":"av1","codec_type":"video"}]}') == ("av1", None)
+
+    def test_garbage_is_not_a_crash(self):
+        assert parse_probe_streams("") == (None, None)
+        assert parse_probe_streams("not json") == (None, None)
+
+
 class TestConcatQuote:
     # PurePosixPath so the escaping is asserted identically on a Windows dev box and in the
     # Linux test container (a WindowsPath would eat the backslash as a separator).