Read a cached source only once a processing slot is taken (closes #64)

A request whose source was in the disk cache read the whole file into
memory, then waited for a processing slot, so a burst of new sizes for
one large cached image held one copy per waiting request, with no
ceiling. The service now opens the cached file and hands it to the image
processor, which reads it only after taking its slot. The file's size,
now returned by GetSourceContent, still sends an empty or oversized
cached source to upstream instead. A cached file that fails while being
read now fails the request instead of being fetched again.

Model: opus-5-5

internal/imgcache/cache.go

@@ -383,13 +383,16 @@ func (c *Cache) GetSourceMetadataID(
 	return id, nil
 }
 
-// GetSourceContent returns a reader for cached source content by its hash.
-func (c *Cache) GetSourceContent(contentHash ContentHash) (io.ReadCloser, error) {
+// GetSourceContent returns a reader for cached source content by its hash,
+// and the content's size in bytes.
+func (c *Cache) GetSourceContent(
+	contentHash ContentHash,
+) (io.ReadCloser, int64, error) {
 	if c.disabled {
-		return nil, ErrNotFound
+		return nil, 0, ErrNotFound
 	}
 
-	return c.srcContent.Load(contentHash)
+	return c.srcContent.LoadWithSize(contentHash)
 }
 
 // CleanExpired removes expired entries from the cache.