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/database/models.go

@@ -16,8 +16,8 @@ type File struct {
 	ID         types.FileID     // UUID primary key
 	Path       types.FilePath   // Absolute path of the file
 	SourcePath types.SourcePath // The source directory this file came from (for restore path stripping)
-	MTime      time.Time
-	CTime      time.Time
+	MTime      time.Time        // Last modification time
+	CTime      time.Time        // Creation/change time (platform-specific: birth time on macOS, inode change time on Linux)
 	Size       int64
 	Mode       uint32
 	UID        uint32