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>

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)