FetchResult now includes:
- StatusCode: HTTP status from upstream
- FetchDurationMs: time to fetch from upstream
- RemoteAddr: upstream server address
SourceMetadata now stores:
- ContentLength: size from upstream
- FetchDurationMs: fetch timing
- RemoteAddr: for debugging
Image conversion log now includes:
- host: source hostname (was missing)
- path: source path (renamed from file)
- convert_ms: image processing time
- quality: requested quality setting
- fit: requested fit mode
- Return ErrUnsupportedOutputFormat for WebP/AVIF encoding
- Return ErrInvalidFitMode for unknown fit mode values
- Add ValidateFitMode() for input validation
- Validate fit mode at handler level before processing
Silent fallbacks violate the principle of least surprise and mask bugs.
When a user explicitly specifies a value, we should either honor it or
return an error - never silently substitute a different value.
