Make the cache stats count what is cached, fetched and transcoded (closes #56)

Cache.Stats read request_cache and output_content, which nothing
writes, so TotalItems and TotalSizeBytes were always 0. They now count
source_content plus variant_content and use UsageBytes; a disabled disk
cache reports 0 for both. The upstream fetch count and bytes and the
transform count never moved: Get now passes the bytes it fetched
(including those read before a failed body read) and counts each
successful transcode. Hits, misses and these counters are written with
context.WithoutCancel, so a client disconnect or the request timeout no
longer loses them. The unused tables stay; metaCache is #70.

Model: opus-5-5

internal/imgcache/cache.go

@@ -419,19 +419,21 @@ func (c *Cache) Stats(ctx context.Context) (*CacheStats, error) {
 		return nil, fmt.Errorf("failed to get cache stats: %w", err)
 	}
 
-	// Get actual item count and total size from content tables
-	err = c.db.QueryRowContext(ctx,
-		`SELECT COUNT(*) FROM request_cache`,
-	).Scan(&stats.TotalItems)
-	if err != nil {
-		c.log.Warn("failed to count cache items for stats", "error", err)
-	}
+	// Count and size the cached source images and processed variants. A
+	// disabled cache holds none, whatever rows an earlier run left.
+	if !c.disabled {
+		err = c.db.QueryRowContext(ctx, `
+			SELECT (SELECT COUNT(*) FROM source_content)
+			     + (SELECT COUNT(*) FROM variant_content)
+		`).Scan(&stats.TotalItems)
+		if err != nil {
+			c.log.Warn("failed to count cache items for stats", "error", err)
+		}
 
-	err = c.db.QueryRowContext(ctx,
-		`SELECT COALESCE(SUM(size_bytes), 0) FROM output_content`,
-	).Scan(&stats.TotalSizeBytes)
-	if err != nil {
-		c.log.Warn("failed to sum cache size for stats", "error", err)
+		stats.TotalSizeBytes, err = c.UsageBytes(ctx)
+		if err != nil {
+			c.log.Warn("failed to sum cache size for stats", "error", err)
+		}
 	}
 
 	// Compute hit rate as a ratio
@@ -481,6 +483,19 @@ func (c *Cache) IncrementStats(ctx context.Context, hit bool, fetchBytes int64)
 	}
 }
 
+// IncrementTransformCount counts one image transcoded by the image processor.
+func (c *Cache) IncrementTransformCount(ctx context.Context) {
+	_, err := c.db.ExecContext(ctx, `
+		UPDATE cache_stats
+		SET transform_count = transform_count + 1,
+		    last_updated_at = CURRENT_TIMESTAMP
+		WHERE id = 1
+	`)
+	if err != nil {
+		c.log.Warn("failed to count transform", "error", err)
+	}
+}
+
 // writeMetadataSidecar writes the JSON metadata sidecar of a stored source.
 // A failure is logged and is otherwise non-fatal; the metadata is in the
 // database.