perf: optimize index build — permanent caches + faster MB rate limit

Three changes to reduce subsequent index build times:

1. Positive image/rels cache TTL: 30 days → 365 days
   Artist url-rels and resolved image URLs rarely change.
   Already-indexed artists make zero API calls on rebuild.

2. Negative cache (misses) stays at 30 days so new images
   are discovered within a month of being added upstream.

3. MB rate limiter for background indexing: 1.0 → 1.5 req/s
   url-rels lookups are lightweight; 1.5/s is well within
   what MB handles (Picard and Kodi both use similar rates).
   Cuts the MB-bound portion of index build by ~33%.

Also adds NewRateLimiterF for fractional rates and caches
MB rels fetch failures (30-day miss TTL) to avoid retrying
unreachable artists every build.

backend/explore/ratelimiter.go

@@ -38,6 +38,14 @@ func NewRateLimiterN(n int) *RateLimiter {
 	}
 }
 
+// NewRateLimiterF returns a rate limiter that allows f requests
+// per second with a burst of 1.
+func NewRateLimiterF(f float64) *RateLimiter {
+	return &RateLimiter{
+		limiter: rate.NewLimiter(rate.Limit(f), 1),
+	}
+}
+
 // Wait blocks until the rate limiter allows the caller to proceed
 // or the context is cancelled. Returns ctx.Err() if the context
 // expires before a token becomes available.