Align with engineering standards (steps 1-5)

- Rename dist/ -> deploy/ with subdirs examples/, scripts/,
  systemd/ per standard repository layout
- Update .gitignore: gitignore all of dist/ (build output only)
- Makefile: all target is now vet->lint->test->build; add vet,
  proto-lint, devserver targets; CGO_ENABLED=0 for builds
  (modernc.org/sqlite is pure-Go, no C toolchain needed);
  CGO_ENABLED=1 retained for tests (race detector)
- Dockerfile: builder -> golang:1.26-alpine, runtime ->
  alpine:3.21; drop libc6 dep; add /srv/mcias/certs and
  /srv/mcias/backups to image
- deploy/systemd/mcias.service: add RestrictSUIDSGID=true
- deploy/systemd/mcias-backup.service: new oneshot backup unit
- deploy/systemd/mcias-backup.timer: daily 02:00 UTC, 5m jitter
- deploy/scripts/install.sh: install backup units and enable
  timer; create certs/ and backups/ subdirs in /srv/mcias
- buf.yaml: add proto linting config for proto-lint target
- internal/db: add Snapshot and SnapshotDir methods (VACUUM INTO)
- cmd/mciasdb: add snapshot subcommand; no master key required

internal/db/snapshot.go

@@ -0,0 +1,68 @@
+package db
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// Snapshot creates a consistent backup of the database at destPath using
+// SQLite's VACUUM INTO statement.  VACUUM INTO acquires a read lock for the
+// duration of the copy, which is safe while the server is running in WAL mode.
+// The destination file is created by SQLite; the caller must ensure the parent
+// directory exists.
+func (db *DB) Snapshot(destPath string) error {
+	// VACUUM INTO is not supported on in-memory databases.
+	if strings.Contains(db.path, "mode=memory") {
+		return fmt.Errorf("db: snapshot not supported on in-memory databases")
+	}
+	if _, err := db.sql.Exec("VACUUM INTO ?", destPath); err != nil {
+		return fmt.Errorf("db: snapshot VACUUM INTO %q: %w", destPath, err)
+	}
+	return nil
+}
+
+// SnapshotDir creates a timestamped backup in dir and prunes backups older
+// than retainDays days.  dir is created with mode 0750 if it does not exist.
+// The backup filename format is mcias-20060102-150405.db.
+func (db *DB) SnapshotDir(dir string, retainDays int) (string, error) {
+	if err := os.MkdirAll(dir, 0750); err != nil {
+		return "", fmt.Errorf("db: create backup dir %q: %w", dir, err)
+	}
+
+	ts := time.Now().UTC().Format("20060102-150405")
+	dest := filepath.Join(dir, fmt.Sprintf("mcias-%s.db", ts))
+	if err := db.Snapshot(dest); err != nil {
+		return "", err
+	}
+
+	// Prune backups older than retainDays.
+	if retainDays > 0 {
+		cutoff := time.Now().UTC().AddDate(0, 0, -retainDays)
+		entries, err := os.ReadDir(dir)
+		if err != nil {
+			// Non-fatal: the backup was written; log pruning failure separately.
+			return dest, fmt.Errorf("db: list backup dir for pruning: %w", err)
+		}
+		for _, e := range entries {
+			if e.IsDir() || !strings.HasSuffix(e.Name(), ".db") {
+				continue
+			}
+			// Skip the file we just wrote.
+			if e.Name() == filepath.Base(dest) {
+				continue
+			}
+			info, err := e.Info()
+			if err != nil {
+				continue
+			}
+			if info.ModTime().Before(cutoff) {
+				_ = os.Remove(filepath.Join(dir, e.Name()))
+			}
+		}
+	}
+
+	return dest, nil
+}