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>

docs/DATAMODEL.md

@@ -17,7 +17,6 @@ Stores metadata about files in the filesystem being backed up.
 - `id` (TEXT PRIMARY KEY) - UUID for the file record
 - `path` (TEXT NOT NULL UNIQUE) - Absolute file path
 - `mtime` (INTEGER NOT NULL) - Modification time as Unix timestamp
-- `ctime` (INTEGER NOT NULL) - Change time as Unix timestamp  
 - `size` (INTEGER NOT NULL) - File size in bytes
 - `mode` (INTEGER NOT NULL) - Unix file permissions and type
 - `uid` (INTEGER NOT NULL) - User ID of file owner