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/ctime_test.go

@@ -0,0 +1,133 @@
+package snapshot
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestFileCTime_RealFile(t *testing.T) {
+	// Create a temporary file
+	dir := t.TempDir()
+	path := filepath.Join(dir, "testfile.txt")
+
+	if err := os.WriteFile(path, []byte("hello"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	info, err := os.Stat(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ctime := fileCTime(info)
+
+	// ctime should be a valid time (not zero)
+	if ctime.IsZero() {
+		t.Fatal("fileCTime returned zero time")
+	}
+
+	// ctime should be close to now (within a few seconds)
+	diff := time.Since(ctime)
+	if diff < 0 || diff > 5*time.Second {
+		t.Fatalf("fileCTime returned unexpected time: %v (diff from now: %v)", ctime, diff)
+	}
+
+	// ctime should not equal mtime exactly in all cases, but for a freshly
+	// created file they should be very close
+	mtime := info.ModTime()
+	ctimeMtimeDiff := ctime.Sub(mtime)
+	if ctimeMtimeDiff < 0 {
+		ctimeMtimeDiff = -ctimeMtimeDiff
+	}
+	// For a freshly created file, ctime and mtime should be within 1 second
+	if ctimeMtimeDiff > time.Second {
+		t.Fatalf("ctime and mtime differ by too much for a new file: ctime=%v, mtime=%v, diff=%v",
+			ctime, mtime, ctimeMtimeDiff)
+	}
+}
+
+func TestFileCTime_AfterMtimeChange(t *testing.T) {
+	// Create a temporary file
+	dir := t.TempDir()
+	path := filepath.Join(dir, "testfile.txt")
+
+	if err := os.WriteFile(path, []byte("hello"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Get initial ctime
+	info1, err := os.Stat(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ctime1 := fileCTime(info1)
+
+	// Change mtime to a time in the past
+	pastTime := time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC)
+	if err := os.Chtimes(path, pastTime, pastTime); err != nil {
+		t.Fatal(err)
+	}
+
+	// Get new stats
+	info2, err := os.Stat(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ctime2 := fileCTime(info2)
+	mtime2 := info2.ModTime()
+
+	// mtime should now be in the past
+	if mtime2.Year() != 2020 {
+		t.Fatalf("mtime not set correctly: %v", mtime2)
+	}
+
+	// On macOS: ctime (birth time) should remain unchanged since birth time
+	// doesn't change when mtime is updated.
+	// On Linux: ctime (inode change time) will be updated to ~now because
+	// changing mtime is a metadata change.
+	// Either way, ctime should NOT equal the past mtime we just set.
+	if ctime2.Equal(pastTime) {
+		t.Fatal("ctime should not equal the artificially set past mtime")
+	}
+
+	// ctime should still be a recent time (the original creation time or
+	// the metadata change time, depending on platform)
+	_ = ctime1 // used for reference; both platforms will have a recent ctime2
+	if time.Since(ctime2) > 10*time.Second {
+		t.Fatalf("ctime is unexpectedly old: %v", ctime2)
+	}
+}
+
+// TestFileCTime_NonSyscallFileInfo verifies the fallback to mtime when
+// the FileInfo doesn't have a *syscall.Stat_t (e.g. afero.MemMapFs).
+type mockFileInfo struct {
+	name    string
+	size    int64
+	mode    os.FileMode
+	modTime time.Time
+	isDir   bool
+}
+
+func (m *mockFileInfo) Name() string       { return m.name }
+func (m *mockFileInfo) Size() int64        { return m.size }
+func (m *mockFileInfo) Mode() os.FileMode  { return m.mode }
+func (m *mockFileInfo) ModTime() time.Time { return m.modTime }
+func (m *mockFileInfo) IsDir() bool        { return m.isDir }
+func (m *mockFileInfo) Sys() interface{}   { return nil } // No syscall.Stat_t
+
+func TestFileCTime_FallbackToMtime(t *testing.T) {
+	now := time.Now().UTC().Truncate(time.Second)
+	info := &mockFileInfo{
+		name:    "test.txt",
+		size:    100,
+		mode:    0644,
+		modTime: now,
+	}
+
+	ctime := fileCTime(info)
+	if !ctime.Equal(now) {
+		t.Fatalf("expected fallback to mtime %v, got %v", now, ctime)
+	}
+}