fix: populate ctime from actual filesystem stats instead of mtime fallback

Replace the mtime fallback for ctime in the scanner with platform-specific
implementations that extract the real ctime from syscall.Stat_t:

- macOS/Darwin: uses Birthtimespec (file birth/creation time)
- Linux: uses Ctim (inode change time)
- Falls back to mtime when syscall stats are unavailable (e.g. afero.MemMapFs)

Also:
- Document platform-specific ctime semantics in README
- Document ctime restore limitations (cannot be set on either platform)
- Add ctime field documentation to File model
- Update README files table schema to match actual schema (adds ctime,
  source_path, link_target columns)
- Add comprehensive tests for fileCTime on real files and mock FileInfo

closes #13

internal/snapshot/backup_test.go

@@ -345,9 +345,9 @@ 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
+			CTime: fileCTime(info), // platform-specific: birth time on macOS, inode change time on Linux
+			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)