Refuse an image URL whose source path or query holds a # (closes #286)

A # written as %23 in the source path or query reached the parser
decoded and stayed in the source URL, where Go reads it as the start of
a fragment and drops it before fetching. So each text after the # was
another unsigned spelling of one upstream request, cached under its own
keys. An image URL whose decoded source path or query holds a # is now
refused with 400, signed or not, and README.md's rule for writing a
source path and query says a # cannot be part of either.

Model: opus-5-5

internal/imgcache/urlparser.go

@@ -171,6 +171,14 @@ func parseImageComponents(remainder string) (*ParsedURL, error) {
 	// host
 	host = strings.ToLower(host)
 
+	// Go's URL parser, which the fetcher uses, reads a "#" as the start of a
+	// fragment and drops the rest, so each text after it would be another
+	// spelling, cached under its own keys, of one upstream request
+	if strings.Contains(path, "#") {
+		return nil, fmt.Errorf(
+			`%w: the source path or query holds a "#"`, ErrInvalidPath)
+	}
+
 	// Extract query string if present in path
 	if qIndex := strings.Index(path, "?"); qIndex != -1 {
 		query = path[qIndex+1:]