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/snapshot/backup_test.go

@@ -345,9 +345,8 @@ func (b *BackupEngine) Backup(ctx context.Context, fsys fs.FS, root string) (str
 			Size:  info.Size(),
 			Mode:  uint32(info.Mode()),
 			MTime: info.ModTime(),
-			CTime: info.ModTime(), // Use mtime as ctime for test
-			UID:   1000,           // Default UID for test
-			GID:   1000,           // Default GID for test
+			UID:   1000, // Default UID for test
+			GID:   1000, // Default GID for test
 		}
 		err = b.repos.WithTx(ctx, func(ctx context.Context, tx *sql.Tx) error {
 			return b.repos.Files.Create(ctx, tx, file)

internal/snapshot/scanner.go

@@ -785,7 +785,6 @@ func (s *Scanner) checkFileInMemory(path string, info os.FileInfo, knownFiles ma
 		Path:       types.FilePath(path),
 		SourcePath: types.SourcePath(s.currentSourcePath), // Store source directory for restore path stripping
 		MTime:      info.ModTime(),
-		CTime:      info.ModTime(), // afero doesn't provide ctime
 		Size:       info.Size(),
 		Mode:       uint32(info.Mode()),
 		UID:        uid,