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/max_concurrent_processing_internal_test.go

@@ -77,11 +77,11 @@ func TestService_Get_WaitsForSlotBeforeReadingCachedSource(t *testing.T) {
 
 	_ = resp.Content.Close()
 
-	contentHash, _, err := svc.cache.LookupSource(t.Context(),
+	sourceKey, _, err := svc.cache.LookupSource(t.Context(),
 		widthOnlyRequest(fixtures, 50))
-	if err != nil || contentHash == "" {
+	if err != nil || sourceKey == "" {
 		t.Fatalf("LookupSource() = %q, %v; want the cached source",
-			contentHash, err)
+			sourceKey, err)
 	}
 
 	release := holdProcessingSlot(t, svc.processor)
@@ -103,7 +103,7 @@ func TestService_Get_WaitsForSlotBeforeReadingCachedSource(t *testing.T) {
 	// before waiting, it would have read it by now.
 	time.Sleep(100 * time.Millisecond)
 
-	err = os.WriteFile(svc.cache.srcContent.hashToPath(contentHash),
+	err = os.WriteFile(svc.cache.srcContent.keyToPath(sourceKey),
 		generateTestJPEG(t, 100, 50, color.RGBA{0, 0, 255, 255}), 0o600)
 	if err != nil {
 		t.Fatalf("failed to rewrite the cached source: %v", err)