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

ARCHITECTURE.md

@@ -54,7 +54,7 @@ The database tracks five primary entities and their relationships:
 
 #### File (`database.File`)
 Represents a file or directory in the backup system. Stores metadata needed for restoration:
-- Path, timestamps (mtime, ctime)
+- Path, mtime
 - Size, mode, ownership (uid, gid)
 - Symlink target (if applicable)