Pace IGDB requests; rewrite the new help text

IGDB allows 4 requests a second, and an import searching three entries
at once (plus detail lookups) went over it. Every IGDB request now waits
at least 300 ms after the last one, and a 429 is retried after 1, 2 and
4 seconds before giving up.

The import dialog, ranking editor and their error messages are reworded
in plainer language, and talk about "spots" instead of "places".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/providers/providers.go

@@ -74,15 +74,19 @@ type Registry struct {
 	tvdb *tvdbTokens
 	igdb *igdbTokens
 	mb   *throttle
+	// IGDB allows 4 requests a second; stay under it so an import's
+	// searches and detail lookups don't get answered with 429.
+	igdbPace *throttle
 }
 
 func NewRegistry(creds func(context.Context) Credentials) *Registry {
 	return &Registry{
-		creds:  creds,
-		client: &http.Client{Timeout: 20 * time.Second},
-		tvdb:   &tvdbTokens{},
-		igdb:   &igdbTokens{},
-		mb:     &throttle{gap: 1100 * time.Millisecond},
+		creds:    creds,
+		client:   &http.Client{Timeout: 20 * time.Second},
+		tvdb:     &tvdbTokens{},
+		igdb:     &igdbTokens{},
+		mb:       &throttle{gap: 1100 * time.Millisecond},
+		igdbPace: &throttle{gap: 300 * time.Millisecond},
 	}
 }