Store a source the way a variant is stored, one row and one file per source URL (closes #308)

A source is now one source_content row and one file named by its
SourceKey, the SHA-256 of its host, path and query, made as the variant
key is. A store writes the file, then upserts the row, and removes the
file when the row cannot be written; an eviction deletes the row, then
the file, with no lock. source_metadata, the JSON files under
cache/metadata/, MetadataStorage, the content lock, the eviction
transaction and its test hook are gone, as is the reconciliation pass
that dropped a source row whose file is missing. Two source URLs
serving the same bytes store them twice.

Model: opus-5-5

internal/imgcache/storage_internal_test.go

@@ -128,7 +128,7 @@ func TestContentStorage_DeleteNonexistent(t *testing.T) {
 	}
 }
 
-func TestContentStorage_KeyToPath(t *testing.T) {
+func TestContentStorage_HashToPath(t *testing.T) {
 	t.Parallel()
 
 	tmpDir := t.TempDir()