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/database/files_test.go

@@ -39,7 +39,6 @@ func TestFileRepository(t *testing.T) {
 	file := &File{
 		Path:       "/test/file.txt",
 		MTime:      time.Now().Truncate(time.Second),
-		CTime:      time.Now().Truncate(time.Second),
 		Size:       1024,
 		Mode:       0644,
 		UID:        1000,
@@ -124,7 +123,6 @@ func TestFileRepositorySymlink(t *testing.T) {
 	symlink := &File{
 		Path:       "/test/link",
 		MTime:      time.Now().Truncate(time.Second),
-		CTime:      time.Now().Truncate(time.Second),
 		Size:       0,
 		Mode:       uint32(0777 | os.ModeSymlink),
 		UID:        1000,
@@ -161,7 +159,6 @@ func TestFileRepositoryTransaction(t *testing.T) {
 		file := &File{
 			Path:  "/test/tx_file.txt",
 			MTime: time.Now().Truncate(time.Second),
-			CTime: time.Now().Truncate(time.Second),
 			Size:  1024,
 			Mode:  0644,
 			UID:   1000,