Eviction no longer reads a whole table while requests wait on the database (closes #227)

UsageBytes now reads the new cache_usage row, which triggers on
source_content and variant_content keep up to date in the statement
that adds, removes or resizes a row. The reconciliation pass reads both
tables 1000 rows per query, sums them, and corrects the total when it
differs, unless a row changed while it summed. Source rows now get
last_accessed_at when added, so choosing source images to evict reads
that column's index instead of sorting the whole table. Stats still
sums the tables, now in pages: an existing test drops both tables and
expects that sum to fail.

Model: opus-5-5

internal/imgcache/cache.go

@@ -96,6 +96,17 @@ type Cache struct {
 	// deterministically pause inside that window to exercise
 	// concurrent stores against it; production code leaves it nil.
 	evictSourceBlobTestHook func(ContentHash)
+
+	// reconciliationPageSize is the most rows one read of a content table
+	// returns in the reconciliation pass and in Stats. newCache sets it to
+	// defaultReconciliationPageSize; tests set it smaller.
+	reconciliationPageSize int
+
+	// reconciliationReadTestHook, when set, is called after each of those
+	// reads with the number of rows the read covered, so tests can check
+	// that no read covers more than one page; production code leaves it
+	// nil.
+	reconciliationReadTestHook func(rows int)
 }
 
 // NewCache creates a new cache instance.
@@ -127,6 +138,8 @@ func newCache(
 		evictionDone:     make(chan struct{}),
 		metaCache:        metaCache,
 		contentLocks:     newContentLock(),
+
+		reconciliationPageSize: defaultReconciliationPageSize,
 	}
 
 	if c.disabled {
@@ -471,8 +484,9 @@ func (c *Cache) Stats(ctx context.Context) (*CacheStats, error) {
 		return nil, fmt.Errorf("failed to get cache stats: %w", err)
 	}
 
-	// Count and size the cached source images and processed variants. A
-	// disabled cache holds none, whatever rows an earlier run left.
+	// Count and size the cached source images and processed variants from
+	// their tables. 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)
@@ -482,7 +496,7 @@ func (c *Cache) Stats(ctx context.Context) (*CacheStats, error) {
 			c.log.Warn("failed to count cache items for stats", "error", err)
 		}
 
-		stats.TotalSizeBytes, err = c.UsageBytes(ctx)
+		stats.TotalSizeBytes, err = c.sumContentSizeBytes(ctx)
 		if err != nil {
 			c.log.Warn("failed to sum cache size for stats", "error", err)
 		}