Check cached sources with upstream after the cache TTL (closes #69)

Cached images were served until eviction, so upstream changes were
missed.

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews it with no download or
conversion. New bytes replace it; older variants are converted again.
A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.

Variant rows keep the source's ETag and Last-Modified for checking a
variant of an evicted source.

A variant file's modification time is its source's check time when it
was made or renewed. Hits read it without a database call. The ETag
includes it, so new bytes get a new ETag.

Model: opus-5-5

internal/imgcache/filestore.go

@@ -3,8 +3,10 @@ package imgcache
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
+	"time"
 )
 
 // Storage constants.
@@ -54,26 +56,26 @@ func (s *FileStore) Write(key string, data []byte) error {
 	return writeFileSynced(path, data)
 }
 
-// Open opens the file for key and returns it with its size in bytes. A
-// missing file is ErrNotFound.
-func (s *FileStore) Open(key string) (*os.File, int64, error) {
+// Open opens the file for key and returns it with its information, which
+// gives its size and its modification time. A missing file is ErrNotFound.
+func (s *FileStore) Open(key string) (*os.File, fs.FileInfo, error) {
 	f, err := os.Open(s.keyToPath(key))
 	if err != nil {
 		if os.IsNotExist(err) {
-			return nil, 0, ErrNotFound
+			return nil, nil, ErrNotFound
 		}
 
-		return nil, 0, fmt.Errorf("failed to open content: %w", err)
+		return nil, nil, fmt.Errorf("failed to open content: %w", err)
 	}
 
-	stat, err := f.Stat()
+	info, err := f.Stat()
 	if err != nil {
 		_ = f.Close()
 
-		return nil, 0, fmt.Errorf("failed to stat content: %w", err)
+		return nil, nil, fmt.Errorf("failed to stat content: %w", err)
 	}
 
-	return f, stat.Size(), nil
+	return f, info, nil
 }
 
 // Size returns the size in bytes of the file for key. A missing file is
@@ -91,6 +93,17 @@ func (s *FileStore) Size(key string) (int64, error) {
 	return stat.Size(), nil
 }
 
+// SetModTime sets the modification time of the file for key to modTime,
+// leaving its access time as it is.
+func (s *FileStore) SetModTime(key string, modTime time.Time) error {
+	err := os.Chtimes(s.keyToPath(key), time.Time{}, modTime)
+	if err != nil {
+		return fmt.Errorf("failed to set modification time: %w", err)
+	}
+
+	return nil
+}
+
 // Delete removes the file for key. A missing file is not an error.
 func (s *FileStore) Delete(key string) error {
 	err := os.Remove(s.keyToPath(key))