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. Two source URLs serving the same
bytes store them twice. Reconciliation still drops a source row whose
file is missing, as TestReconciliationReadsAPageAtATime counts those
reads. The tests of the removed sharing, lock and hook go with them.

Model: opus-5-5

internal/database/concurrent_writes_internal_test.go

@@ -131,7 +131,8 @@ func writeLikeOneRequest(ctx context.Context, db *sql.DB, key string) error {
 	}
 
 	_, err = db.ExecContext(ctx, `INSERT INTO source_content
-		(content_hash, content_type, size_bytes) VALUES (?, 'image/png', 1)`, key)
+		(source_key, source_host, source_path, content_type, size_bytes)
+		VALUES (?, 'example.com', '/image.png', 'image/png', 1)`, key)
 	if err != nil {
 		return fmt.Errorf("storing a source: %w", err)
 	}