Read/write config files as UTF-8 on Windows (#4)

Path.read_text()/write_text() default to the platform encoding (cp1252
on Windows), which mangles or crashes on UTF-8 files. yt-dlp's
video.info.json is UTF-8 and routinely contains non-ASCII bytes, so on
Windows the parse raised UnicodeDecodeError, was swallowed by a bare
except, and the /watch report came back missing Title/Uploader.

- download.py: read video.info.json as UTF-8; log parse failures to
  stderr instead of swallowing them silently.
- whisper.py, setup.py: same UTF-8 fix on .env reads and writes for
  symmetry across platforms.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/download.py

@@ -98,14 +98,15 @@ def download_url(url: str, out_dir: Path) -> dict:
     info: dict = {}
     if info_path.exists():
         try:
-            raw = json.loads(info_path.read_text())
+            raw = json.loads(info_path.read_text(encoding="utf-8"))
             info = {
                 "title": raw.get("title"),
                 "uploader": raw.get("uploader") or raw.get("channel"),
                 "duration": raw.get("duration"),
                 "url": raw.get("webpage_url") or url,
             }
-        except Exception:
+        except Exception as exc:
+            print(f"[watch] info.json parse failed: {exc}", file=sys.stderr)
             info = {"url": url}
 
     return {