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/schema.sql

@@ -8,7 +8,6 @@ CREATE TABLE IF NOT EXISTS files (
     path TEXT NOT NULL UNIQUE,
     source_path TEXT NOT NULL DEFAULT '',  -- The source directory this file came from (for restore path stripping)
     mtime INTEGER NOT NULL,
-    ctime INTEGER NOT NULL,
     size INTEGER NOT NULL,
     mode INTEGER NOT NULL,
     uid INTEGER NOT NULL,