fix(midi): route MIDI live via masterBus, fix media explorer drag, add action logging

- TrackInstrument: gate native live path (SF/VSTi WASAPI bridge bypassed
  masterBus -> mastering FX + main out VU dead). Fallback to SonicSF WASM
  through masterBus.input; native kept for offline render only.
- MEDIA_LIBRARY_SAMPLES: rename MIDI_Loop_02_Bass/03_Lead/05_Bass to match
  actual /static/midi files (drag was 404 -> parse error).
- Add SonicAppLogger (console + POST /api/v1/logs) wired to: instrument load
  error, VSTi autosample fail, MIDI item -> mastering fx chain, play -> main
  out, drag MIDI errors (media explorer + window explorer).
- Add backend /api/v1/logs endpoint (server-side log, no DB).

app/api/v1/logs.py

@@ -0,0 +1,39 @@
+# SonicForge Client Logs API — client gửi log hành động (MIDI/mastering/drag
+# errors) qua POST /api/v1/logs. Không lưu DB — in ra server log (uvicorn
+# console). Public: log không chứa dữ liệu nhạy (client tự quyết định).
+import logging
+from typing import Any, Optional
+from fastapi import APIRouter
+from pydantic import BaseModel
+
+router = APIRouter()
+_logger = logging.getLogger("sonicforge.client_logs")
+
+
+class ClientLogEntry(BaseModel):
+    category: str = "GENERAL"
+    level: str = "info"
+    message: str = ""
+    data: Optional[dict[str, Any]] = None
+    url: Optional[str] = None
+    ts: Optional[str] = None
+
+
+@router.post("")
+async def post_client_log(entry: ClientLogEntry):
+    """Ghi log từ client vào server log (fire-and-forget, luôn 200)."""
+    tag = f"[ClientLog][{entry.category}][{entry.level}]"
+    line = f"{tag} {entry.message}"
+    if entry.data:
+        line += f" {entry.data}"
+    if entry.ts:
+        line += f" (ts={entry.ts})"
+    if entry.url:
+        line += f" url={entry.url}"
+    if entry.level == "error":
+        _logger.error(line)
+    elif entry.level == "warn":
+        _logger.warning(line)
+    else:
+        _logger.info(line)
+    return {"ok": True}