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/cache_internal_test.go

@@ -24,26 +24,14 @@ func setupTestDB(t *testing.T) *sql.DB {
 	// Create schema
 	schema := `
 		CREATE TABLE source_content (
-			content_hash TEXT PRIMARY KEY,
-			content_type TEXT NOT NULL,
-			size_bytes INTEGER NOT NULL,
-			fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP
-		);
-		CREATE TABLE source_metadata (
-			id INTEGER PRIMARY KEY AUTOINCREMENT,
+			source_key TEXT PRIMARY KEY,
 			source_host TEXT NOT NULL,
 			source_path TEXT NOT NULL,
 			source_query TEXT NOT NULL DEFAULT '',
-			path_hash TEXT NOT NULL,
-			content_hash TEXT,
-			status_code INTEGER NOT NULL,
-			content_type TEXT,
-			response_headers TEXT,
+			content_type TEXT NOT NULL,
+			size_bytes INTEGER NOT NULL,
 			fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP,
-			expires_at DATETIME,
-			etag TEXT,
-			last_modified TEXT,
-			UNIQUE(source_host, source_path, source_query)
+			last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 		);
 		CREATE TABLE output_content (
 			content_hash TEXT PRIMARY KEY,
@@ -171,14 +159,14 @@ func TestCache_StoreAndLookup(t *testing.T) {
 		Headers:     map[string][]string{testHeaderContentType: {testContentTypeJPEG}},
 	}
 
-	contentHash, err := cache.StoreSource(
+	sourceKey, err := cache.StoreSource(
 		ctx, req, bytes.NewReader(sourceContent), fetchResult)
 	if err != nil {
 		t.Fatalf("StoreSource() error = %v", err)
 	}
 
-	if contentHash == "" {
-		t.Error("StoreSource() returned empty hash")
+	if sourceKey == "" {
+		t.Error("StoreSource() returned empty key")
 	}
 
 	// Store variant
@@ -547,7 +535,6 @@ func TestCache_StorageDirectoriesCreated(t *testing.T) {
 	dirs := []string{
 		"cache/sources",
 		"cache/variants",
-		"cache/metadata",
 	}
 
 	for _, dir := range dirs {