Share one fetch and transcode among concurrent misses (closes #65)

Requests that missed the same variant at once each fetched and
transcoded it. They now share one call through
golang.org/x/sync/singleflight, keyed on the variant cache key. The
first request processes the variant with a context that does not end
with its own; the others wait for its result, holding no connection or
processing slot, and return as soon as their own context ends. The
processing request waits even then, as before. Each request counts one
miss; the processing counts its fetch and transcode once. A panic
while processing becomes an error instead of stopping pixad.

Model: opus-5-5

internal/imgcache/cache.go

@@ -447,7 +447,8 @@ func (c *Cache) Stats(ctx context.Context) (*CacheStats, error) {
 	return &stats, nil
 }
 
-// IncrementStats increments cache statistics.
+// IncrementStats counts a cache hit or miss, and an upstream fetch that read
+// fetchBytes bytes, as IncrementUpstreamFetch does.
 func (c *Cache) IncrementStats(ctx context.Context, hit bool, fetchBytes int64) {
 	var err error
 
@@ -471,18 +472,26 @@ func (c *Cache) IncrementStats(ctx context.Context, hit bool, fetchBytes int64)
 		c.log.Warn("failed to count cache hit or miss", "hit", hit, "error", err)
 	}
 
-	if fetchBytes > 0 {
-		_, err = c.db.ExecContext(ctx, `
-			UPDATE cache_stats
-			SET upstream_fetch_count = upstream_fetch_count + 1,
-			    upstream_fetch_bytes = upstream_fetch_bytes + ?,
-			    last_updated_at = CURRENT_TIMESTAMP
-			WHERE id = 1
-		`, fetchBytes)
-		if err != nil {
-			c.log.Warn("failed to count upstream fetch",
-				"fetch_bytes", fetchBytes, "error", err)
-		}
+	c.IncrementUpstreamFetch(ctx, fetchBytes)
+}
+
+// IncrementUpstreamFetch counts one upstream fetch that read fetchBytes bytes.
+// A fetch that read no bytes is not counted.
+func (c *Cache) IncrementUpstreamFetch(ctx context.Context, fetchBytes int64) {
+	if fetchBytes <= 0 {
+		return
+	}
+
+	_, err := c.db.ExecContext(ctx, `
+		UPDATE cache_stats
+		SET upstream_fetch_count = upstream_fetch_count + 1,
+		    upstream_fetch_bytes = upstream_fetch_bytes + ?,
+		    last_updated_at = CURRENT_TIMESTAMP
+		WHERE id = 1
+	`, fetchBytes)
+	if err != nil {
+		c.log.Warn("failed to count upstream fetch",
+			"fetch_bytes", fetchBytes, "error", err)
 	}
 }