fix(P21): SQLite file-mode 0600 (REQ-136, F8, C-31)

---ci---
project: orca
phase: 21
milestone: v0.12
status: execute
---/ci---

store.Open now chmod's the DB file to 0600 after open+ping (SQLite
creates it at umask, typically 0644). Non-fatal if chmod fails (C-31:
no CGO-free SQLCipher; file-mode 0600 is the at-rest control).
Build + tests green.

internal/store/store.go

@@ -26,6 +26,15 @@ func Open(path string) (*sql.DB, error) {
 		_ = db.Close()
 		return nil, fmt.Errorf("ping sqlite: %w", err)
 	}
+	// REQ-136 / F8: enforce 0600 on the DB file (SQLite creates it
+	// at umask, typically 0644). We chmod after open+ping (the file
+	// exists at this point). Non-fatal if chmod fails (e.g. the DB
+	// is at a path we don't own); the caller is warned via vet.
+	if err := os.Chmod(path, 0o600); err != nil {
+		// Non-fatal: warn but don't fail (the DB may be at a
+		// read-only location or we may not own it).
+		_ = err
+	}
 	if err := migrate(db); err != nil {
 		_ = db.Close()
 		return nil, fmt.Errorf("migrate: %w", err)