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/whisper.py

@@ -45,7 +45,7 @@ def load_api_key(preferred: str | None = None) -> tuple[str, str] | tuple[None,
         if not path.exists():
             return None
         try:
-            for line in path.read_text().splitlines():
+            for line in path.read_text(encoding="utf-8").splitlines():
                 line = line.strip()
                 if not line or line.startswith("#") or "=" not in line:
                     continue