fix(export): MIDI export qua server endpoint thay blob download (WebView2 chặn) + velocity 1..127, bỏ rest-track

app/api/v1/audio.py

@@ -56,6 +56,21 @@ class ExportRequest(BaseModel):
     sample_rate: int = 44100
     bit_depth: int = 16
 
+class MidiNoteModel(BaseModel):
+    pitch: int = 60
+    start_beat: float = 0.0
+    duration_beats: float = 1.0
+    velocity: float = 0.8  # 0..1
+
+class MidiExportTrackModel(BaseModel):
+    name: str = "Track"
+    notes: List[MidiNoteModel] = []
+
+class MidiExportRequest(BaseModel):
+    bpm: float = 120.0
+    ppq: int = 480
+    tracks: List[MidiExportTrackModel] = []
+
 class AIAnalysisRequest(BaseModel):
     file_id: str
     api_base_url: Optional[str] = None
@@ -153,7 +168,8 @@ async def download_audio(file_id: str):
     path = _resolve_storage_path(file_id)
     if not path:
         raise HTTPException(status_code=404, detail="File not found")
-    return FileResponse(path, media_type="audio/wav", filename=os.path.basename(path))
+    media = "audio/midi" if os.path.splitext(path)[1].lower() == ".mid" else "audio/wav"
+    return FileResponse(path, media_type=media, filename=os.path.basename(path))
 
 @router.get("/waveform/{file_id}")
 async def get_waveform(file_id: str, num_peaks: int = Query(default=800, ge=50, le=4000)):
@@ -226,6 +242,73 @@ async def export_audio(req: ExportRequest, current_user: Optional[dict] = Depend
         "file_id": _safe_file_id(req.file_id)
     }
 
+# ── MIDI export (BIG_PHASE_3 Phase 1) ──
+# Sinh file .mid format 1 bằng stdlib thuần — trước đây frontend tự build bytes
+# rồi blob-download (bị WebView2 chặn sau khi chuyển standalone). Server sinh
+# file → GET /download/{file_id} (network download, pattern đã chạy ổn định).
+def _write_vlq(data: bytearray, value: int) -> None:
+    buf = [value & 0x7F]
+    while value > 0x7F:
+        value >>= 7
+        buf.append(0x80 | (value & 0x7F))
+    for b in reversed(buf):
+        data.append(b)
+
+def _build_midi_file(req: MidiExportRequest) -> bytes:
+    import struct
+    tpb = max(1, req.ppq or 480)
+    track_chunks = []
+    for tr in req.tracks:
+        events = []
+        for n in tr.notes:
+            if (n.duration_beats or 0) <= 0:
+                continue
+            pitch = max(0, min(127, int(round(n.pitch or 60))))
+            vel = max(1, min(127, int(round((n.velocity if n.velocity is not None else 0.8) * 127))))
+            start = max(0, int(round((n.start_beat or 0) * tpb)))
+            end = start + max(1, int(round((n.duration_beats or 1) * tpb)))
+            events.append((start, 0x90, pitch, vel))
+            events.append((end, 0x80, pitch, 0))
+        if not events:
+            continue  # bỏ track không có nốt (trước đây chèn "rest note" pitch 0)
+        # note_off (0x80) trước note_on (0x90) cùng tick — tránh note kẹt
+        events.sort(key=lambda e: (e[0], 0 if e[1] == 0x80 else 1))
+        body = bytearray()
+        name_bytes = (tr.name or "Track").encode("utf-8", "replace")[:255]
+        body.append(0xFF); body.append(0x03); body.append(len(name_bytes)); body.extend(name_bytes)
+        last = 0
+        for tick, status, pitch, vel in events:
+            _write_vlq(body, tick - last)
+            last = tick
+            body.append(status); body.append(pitch); body.append(vel)
+        _write_vlq(body, 0)
+        body.append(0xFF); body.append(0x2F); body.append(0x00)
+        chunk = bytearray(b"MTrk")
+        chunk += struct.pack(">I", len(body))
+        chunk += body
+        track_chunks.append(bytes(chunk))
+    if not track_chunks:
+        return b""
+    header = bytearray(b"MThd")
+    header += struct.pack(">IHHH", 6, 1, len(track_chunks), tpb)
+    return bytes(header) + b"".join(track_chunks)
+
+@router.post("/export/midi")
+async def export_midi(req: MidiExportRequest, current_user: Optional[dict] = Depends(get_optional_user)):
+    if current_user:
+        enforce_password_changed(current_user)
+    data = _build_midi_file(req)
+    if not data:
+        raise HTTPException(status_code=400, detail="Không có dữ liệu MIDI nào để xuất")
+    file_id = "midi_export_%s.mid" % uuid.uuid4().hex[:12]
+    with open(os.path.join(settings.PROCESSED_DIR, file_id), "wb") as f:
+        f.write(data)
+    return {
+        "file_id": file_id,
+        "download_url": "/api/v1/audio/download/%s" % file_id,
+        "bytes": len(data)
+    }
+
 @router.post("/ai-scan")
 async def ai_scan_audio(req: AIScanRequest, current_user: Optional[dict] = Depends(get_optional_user)):
     """