fix: populate ctime from platform-specific syscall data

The scanner was setting CTime to info.ModTime() as a placeholder since
afero's FileInfo interface doesn't expose ctime directly. This change
extracts the actual ctime from the underlying syscall.Stat_t via
platform-specific build files:

- macOS (Darwin): uses Birthtimespec (file creation/birth time)
- Linux: uses Ctim (inode change time)
- Other platforms: falls back to mtime

Also adds:
- Documentation of ctime semantics in README.md (new 'file metadata' section)
- Platform differences table (macOS birth time vs Linux inode change time)
- Note that ctime is recorded but not restored (not settable via standard APIs)
- Updated README schema to match actual schema (adds ctime, source_path, link_target)
- Doc comment on CTime field in database model

closes #13

internal/snapshot/ctime_other.go

@@ -0,0 +1,15 @@
+//go:build !darwin && !linux
+
+package snapshot
+
+import (
+	"os"
+	"time"
+)
+
+// getCTime returns the file's modification time as a fallback on unsupported
+// platforms. See ctime_darwin.go and ctime_linux.go for platform-specific
+// implementations that extract actual ctime/birth time from syscall data.
+func getCTime(info os.FileInfo) time.Time {
+	return info.ModTime()
+}