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/repository_debug_test.go

@@ -18,7 +18,6 @@ func TestOrphanedFileCleanupDebug(t *testing.T) {
 	file1 := &File{
 		Path:  "/orphaned.txt",
 		MTime: time.Now().Truncate(time.Second),
-		CTime: time.Now().Truncate(time.Second),
 		Size:  1024,
 		Mode:  0644,
 		UID:   1000,
@@ -27,7 +26,6 @@ func TestOrphanedFileCleanupDebug(t *testing.T) {
 	file2 := &File{
 		Path:  "/referenced.txt",
 		MTime: time.Now().Truncate(time.Second),
-		CTime: time.Now().Truncate(time.Second),
 		Size:  2048,
 		Mode:  0644,
 		UID:   1000,