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

cmd/mciasdb/snapshot.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"path/filepath"
+
+	"git.wntrmute.dev/kyle/mcias/internal/config"
+	"git.wntrmute.dev/kyle/mcias/internal/db"
+)
+
+// runSnapshot handles the "snapshot" command.
+//
+// It opens the database read-only (no master key derivation needed — VACUUM
+// INTO does not access encrypted columns) and writes a timestamped backup to
+// /srv/mcias/backups/ (or the directory adjacent to the configured DB path).
+// Backups older than --retain-days are pruned.
+func runSnapshot(configPath string, args []string) {
+	fs := flag.NewFlagSet("snapshot", flag.ExitOnError)
+	retainDays := fs.Int("retain-days", 30, "prune backups older than this many days (0 = keep all)")
+	if err := fs.Parse(args); err != nil {
+		fatalf("snapshot: %v", err)
+	}
+
+	cfg, err := config.Load(configPath)
+	if err != nil {
+		fatalf("snapshot: load config: %v", err)
+	}
+
+	database, err := db.Open(cfg.Database.Path)
+	if err != nil {
+		fatalf("snapshot: open database: %v", err)
+	}
+	defer func() { _ = database.Close() }()
+
+	// Place backups in a "backups" directory adjacent to the database file.
+	backupDir := filepath.Join(filepath.Dir(cfg.Database.Path), "backups")
+
+	dest, err := database.SnapshotDir(backupDir, *retainDays)
+	if err != nil {
+		fatalf("snapshot: %v", err)
+	}
+	fmt.Printf("snapshot written: %s\n", dest)
+}