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
- Manifests are now only compressed (not encrypted) so pruning operations can work without private keys
- Updated generateBlobManifest to use zstd compression directly
- Updated prune command to handle unencrypted manifests
- Updated snapshot list command to handle new manifest format
- Updated documentation to reflect manifest.json.zst (not .age)
- Removed unnecessary VAULTIK_PRIVATE_KEY check from prune command
