fix(signal): detect M4A-branded voice notes so iOS audio reaches STT

iOS Signal delivers voice notes as MP4-container AAC carrying an audio
ftyp brand ("M4A "). `_guess_extension()` returned ".mp4" for every
`ftyp` file regardless of brand, so those attachments were cached as
documents instead of audio and STT rejected the upload:

    API error: Error code: 400 - Invalid file format.
    Supported formats: ['flac','m4a','mp3','mp4','mpeg','mpga','oga',
    'ogg','wav','webm']

Read the 4-byte brand at offset 8 and return ".m4a" for audio brands
("M4A ", "M4B ") so they satisfy `_is_audio_ext()` and route to
`cache_audio_from_bytes()`. Video brands (isom/mp42/avc1/qt) still
return ".mp4".

This mirrors the existing brand/form-type disambiguation already used
in this function for RIFF (WEBP vs WAVE) and for ADTS AAC vs MP3.

Verified against a real iOS Signal voice note: pre-fix the upload was
rejected with the 400 above; post-fix the same bytes transcribe
successfully.

gateway/platforms/signal.py

@@ -106,6 +106,14 @@ def _guess_extension(data: bytes) -> str:
     if data[:4] == b"%PDF":
         return ".pdf"
     if len(data) >= 8 and data[4:8] == b"ftyp":
+        # iOS Signal delivers voice notes as MP4-container AAC carrying an
+        # audio ftyp brand ("M4A ", "M4B "). Returning ".mp4" for those made
+        # them cache as documents and STT reject them ("Invalid file
+        # format") even though the bytes are valid audio. Read the brand so
+        # audio-branded files land as ".m4a" and route to the audio cache.
+        brand = data[8:12].lower() if len(data) >= 12 else b""
+        if brand in (b"m4a ", b"m4b "):
+            return ".m4a"
         return ".mp4"
     if data[:4] == b"OggS":
         return ".ogg"

tests/gateway/test_signal.py

@@ -201,6 +201,26 @@ class TestSignalHelpers:
         webp = b"RIFF\x24\x08\x00\x00WEBPVP8 " + b"\x00" * 100
         assert _guess_extension(webp) == ".webp"
 
+    def test_guess_extension_m4a_audio_brand(self):
+        """iOS Signal voice notes are MP4-container AAC with an M4A ftyp brand.
+
+        Classifying them as ``.mp4`` sent them to the document cache and made
+        STT reject the upload ("Invalid file format") even though the bytes
+        were valid audio. Audio brands must resolve to ``.m4a``.
+        """
+        from gateway.platforms.signal import _guess_extension, _is_audio_ext
+        for brand in (b"M4A ", b"M4B ", b"m4a "):
+            data = b"\x00\x00\x00\x1cftyp" + brand + b"\x00" * 100
+            assert _guess_extension(data) == ".m4a", brand
+            assert _is_audio_ext(_guess_extension(data)) is True
+
+    def test_guess_extension_video_brands_stay_mp4(self):
+        """Real video ftyp brands must not be swept up by the M4A fix."""
+        from gateway.platforms.signal import _guess_extension
+        for brand in (b"isom", b"mp42", b"avc1", b"qt  "):
+            data = b"\x00\x00\x00\x1cftyp" + brand + b"\x00" * 100
+            assert _guess_extension(data) == ".mp4", brand
+
     def test_guess_extension_aac_adts_unprotected(self):
         """ADTS AAC, MPEG-4, no CRC (the canonical Android Signal voice note).