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 ignores its
cancellation but keeps its deadline; 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, up to its
deadline. Each request counts one miss; the processing counts its fetch
and transcode once. A panic while processing is reported through the
request's Sentry hub, when there is one, and becomes an error instead
of stopping pixad.

Model: opus-5-5

internal/imgcache/cache.go

@@ -471,7 +471,8 @@ 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
 
@@ -495,18 +496,26 @@ 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)
 	}
 }