Standalone Shonar Desktop: vendor portable sources + local engine; decouple from ~/Projects/Shonar

- shared/ = portable Android-origin sources vendored from deferred/desktop-server
  (app/build.gradle.kts srcDir repointed; PlaybackController.kt excluded as Android-only)
- backend/ = bundled-lite engine (SQLite + inline queue); .venv symlinked from the
  old checkout, PYTHONPATH pins THIS backend's code over any editable install
- repoRoot() resolves this project dir (env SHONAR_REPO still wins); desktop-dev.sh
  watches shared/ + backend/
- Verified: :app:compileKotlin + :app:test green (23 tests); engine boots on :8010,
  self-migrates, /healthz ok

backend/shonar/services/media.py

@@ -0,0 +1,74 @@
+"""Audio format validation: declared MIME type vs magic bytes.
+
+Families decouple declared MIME aliases (audio/mp4 vs audio/m4a) from what
+the bytes actually are. The original file is stored exactly as uploaded —
+validation never rewrites it.
+"""
+
+from __future__ import annotations
+
+FAMILY_BY_MIME = {
+    "audio/mp4": "mp4",
+    "audio/m4a": "mp4",
+    "audio/aac": "aac",
+    "audio/wav": "wav",
+    "audio/x-wav": "wav",
+    "audio/ogg": "ogg",
+    "audio/opus": "ogg",
+    "audio/webm": "webm",
+    "audio/mpeg": "mpeg",
+}
+
+EXT_BY_FAMILY = {
+    "mp4": ".m4a",
+    "aac": ".aac",
+    "wav": ".wav",
+    "ogg": ".ogg",
+    "webm": ".webm",
+    "mpeg": ".mp3",
+}
+
+
+def sniff_audio_family(data: bytes) -> str | None:
+    """Return the audio family from magic bytes, or None if unrecognized."""
+    if len(data) >= 12:
+        if data[4:8] == b"ftyp":
+            return "mp4"
+        if data[:4] == b"RIFF" and data[8:12] == b"WAVE":
+            return "wav"
+    if data[:4] == b"OggS":
+        return "ogg"
+    if data[:4] == b"\x1a\x45\xdf\xa3":
+        return "webm"
+    if data[:3] == b"ID3":
+        return "mpeg"
+    if len(data) >= 2 and data[0] == 0xFF and (data[1] & 0xF6) == 0xF0:
+        # ADTS frame sync: AAC (also accepted as mpeg-family audio)
+        return "aac"
+    return None
+
+
+def declared_family(mime_type: str) -> str | None:
+    return FAMILY_BY_MIME.get(mime_type.lower().split(";")[0].strip())
+
+
+def is_compatible(mime_type: str, data: bytes) -> bool:
+    """True when the declared MIME matches the sniffed magic bytes.
+
+    mpeg and aac are treated as one family: Android records AAC in ADTS or
+    in MP4 containers and MIME reporting around these is inconsistent.
+    """
+    declared = declared_family(mime_type)
+    sniffed = sniff_audio_family(data)
+    if declared is None or sniffed is None:
+        return False
+    if {declared, sniffed} == {"mpeg", "aac"}:
+        return True
+    return declared == sniffed
+
+
+def extension_for(mime_type: str, data: bytes) -> str:
+    sniffed = sniff_audio_family(data)
+    if sniffed is not None:
+        return EXT_BY_FAMILY[sniffed]
+    return EXT_BY_FAMILY.get(declared_family(mime_type) or "", ".bin")