fix: populate ctime from actual filesystem stats instead of mtime fallback

Replace the mtime fallback for ctime in the scanner with platform-specific
implementations that extract the real ctime from syscall.Stat_t:

- macOS/Darwin: uses Birthtimespec (file birth/creation time)
- Linux: uses Ctim (inode change time)
- Falls back to mtime when syscall stats are unavailable (e.g. afero.MemMapFs)

Also:
- Document platform-specific ctime semantics in README
- Document ctime restore limitations (cannot be set on either platform)
- Add ctime field documentation to File model
- Update README files table schema to match actual schema (adds ctime,
  source_path, link_target columns)
- Add comprehensive tests for fileCTime on real files and mock FileInfo

closes #13

README.md

@@ -194,8 +194,9 @@ vaultik [--config <path>] store info
 * Requires `VAULTIK_AGE_SECRET_KEY` environment variable with age private key
 * Optional path arguments to restore specific files/directories (default: all)
 * Downloads and decrypts metadata, fetches required blobs, reconstructs files
-* Preserves file permissions, timestamps, and ownership (ownership requires root)
+* Preserves file permissions, mtime, and ownership (ownership requires root)
 * Handles symlinks and directories
+* Note: ctime cannot be restored (see [platform notes](#platform-specific-ctime-semantics))
 
 **prune**: Remove unreferenced blobs from remote storage
 * Scans all snapshots for referenced blobs
@@ -247,11 +248,14 @@ Snapshot IDs follow the format `<hostname>_<snapshot-name>_<timestamp>` (e.g., `
 CREATE TABLE files (
   id TEXT PRIMARY KEY,
   path TEXT NOT NULL UNIQUE,
+  source_path TEXT NOT NULL DEFAULT '',
   mtime INTEGER NOT NULL,
+  ctime INTEGER NOT NULL,
   size INTEGER NOT NULL,
   mode INTEGER NOT NULL,
   uid INTEGER NOT NULL,
-  gid INTEGER NOT NULL
+  gid INTEGER NOT NULL,
+  link_target TEXT
 );
 
 CREATE TABLE file_chunks (
@@ -339,7 +343,25 @@ CREATE TABLE snapshot_blobs (
 1. For each file, get ordered chunk list from file_chunks
 1. Download required blobs, decrypt, decompress
 1. Extract chunks and reconstruct files
-1. Restore permissions, mtime, uid/gid
+1. Restore permissions, mtime, uid/gid (ctime cannot be restored — see platform notes above)
+
+### platform-specific ctime semantics
+
+The `ctime` field in the files table stores a platform-dependent timestamp:
+
+* **macOS (Darwin)**: `ctime` is the file's **birth time** — when the file was
+  first created on disk. This value never changes after file creation, even if
+  the file's content or metadata is modified.
+
+* **Linux**: `ctime` is the **inode change time** — the last time the file's
+  metadata (permissions, ownership, link count, etc.) was modified. This is NOT
+  the file creation time. Linux did not expose birth time (via `statx(2)`) until
+  kernel 4.11, and Go's `syscall` package does not yet surface it.
+
+**Restore limitation**: `ctime` cannot be restored on either platform. On Linux,
+the kernel manages the inode change time and userspace cannot set it. On macOS,
+there is no standard POSIX API to set birth time. The `ctime` value is preserved
+in the snapshot database for informational/forensic purposes only.
 
 #### prune