Remove all ctime usage and storage (#55)

Remove all ctime from the codebase per sneak's decision on [PR #48](#48).

## Rationale

- ctime means different things on macOS (birth time) vs Linux (inode change time) — ambiguous cross-platform
- Vaultik never uses ctime operationally (scanning triggers on mtime change)
- Cannot be restored on either platform
- Write-only forensic data with no consumer

## Changes

- **Schema** (`internal/database/schema.sql`): Removed `ctime` column from `files` table
- **Model** (`internal/database/models.go`): Removed `CTime` field from `File` struct
- **Database layer** (`internal/database/files.go`): Removed ctime from all INSERT/SELECT queries, ON CONFLICT updates, and scan targets in both `scanFile` and `scanFileRows` helpers; updated `CreateBatch` accordingly
- **Scanner** (`internal/snapshot/scanner.go`): Removed `CTime: info.ModTime()` assignment in `checkFileInMemory()`
- **Tests**: Removed all `CTime` field assignments from 8 test files
- **Documentation**: Removed ctime references from `ARCHITECTURE.md` and `docs/DATAMODEL.md`

`docker build .` passes clean (lint, fmt-check, all tests).

closes #54

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #55
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/snapshot/backup_test.go

@@ -345,9 +345,8 @@ func (b *BackupEngine) Backup(ctx context.Context, fsys fs.FS, root string) (str
 			Size:  info.Size(),
 			Mode:  uint32(info.Mode()),
 			MTime: info.ModTime(),
-			CTime: info.ModTime(), // Use mtime as ctime for test
-			UID:   1000,           // Default UID for test
-			GID:   1000,           // Default GID for test
+			UID:   1000, // Default UID for test
+			GID:   1000, // Default GID for test
 		}
 		err = b.repos.WithTx(ctx, func(ctx context.Context, tx *sql.Tx) error {
 			return b.repos.Files.Create(ctx, tx, file)