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

deploy/systemd/mcias-backup.service

@@ -0,0 +1,32 @@
+[Unit]
+Description=MCIAS Database Backup
+Documentation=man:mciasdb(1)
+After=mcias.service
+# Backup runs against the live database using VACUUM INTO, which is safe
+# while mciassrv is running (WAL mode allows concurrent readers).
+
+[Service]
+Type=oneshot
+User=mcias
+Group=mcias
+
+EnvironmentFile=/srv/mcias/env
+
+ExecStart=/usr/local/bin/mciasdb -config /srv/mcias/mcias.toml snapshot
+
+# Filesystem restrictions (read-write to /srv/mcias for the backup output).
+ProtectSystem=strict
+ProtectHome=true
+PrivateTmp=true
+ReadWritePaths=/srv/mcias
+NoNewPrivileges=true
+PrivateDevices=true
+CapabilityBoundingSet=
+RestrictSUIDSGID=true
+RestrictNamespaces=true
+RestrictRealtime=true
+LockPersonality=true
+MemoryDenyWriteExecute=true
+ProtectKernelTunables=true
+ProtectKernelModules=true
+ProtectControlGroups=true