remove ctime column from schema, model, queries, scanner, and docs

ctime is ambiguous cross-platform (macOS birth time vs Linux inode change
time), never used operationally (scanning triggers on mtime), cannot be
restored on either platform, and was write-only forensic data with no
consumer.

Removes ctime from:
- files table schema (schema.sql)
- File struct (models.go)
- all SQL queries and scan targets (files.go)
- scanner file metadata collection (scanner.go)
- all test files
- ARCHITECTURE.md and docs/DATAMODEL.md

closes #54

internal/database/file_chunks_test.go

@@ -22,7 +22,6 @@ func TestFileChunkRepository(t *testing.T) {
 	file := &File{
 		Path:       "/test/file.txt",
 		MTime:      testTime,
-		CTime:      testTime,
 		Size:       3072,
 		Mode:       0644,
 		UID:        1000,
@@ -135,7 +134,6 @@ func TestFileChunkRepositoryMultipleFiles(t *testing.T) {
 		file := &File{
 			Path:       types.FilePath(path),
 			MTime:      testTime,
-			CTime:      testTime,
 			Size:       2048,
 			Mode:       0644,
 			UID:        1000,