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/models.go

@@ -17,7 +17,6 @@ type File struct {
 	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
 	Size       int64
 	Mode       uint32
 	UID        uint32