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/examples/mcias.env.example

@@ -0,0 +1,17 @@
+# /srv/mcias/env — Environment file for mciassrv (systemd EnvironmentFile).
+#
+# This file is loaded by the mcias.service unit before the server starts.
+# It must be readable only by root and the mcias service account:
+#
+#   chmod 0640 /srv/mcias/env
+#   chown root:mcias /srv/mcias/env
+#
+# SECURITY: This file contains the master key passphrase. Treat it with
+# the same care as a private key. Do not commit it to version control.
+# Back it up to a secure offline location — losing this passphrase means
+# losing access to all encrypted data in the database.
+
+# Master key passphrase. Used to derive the AES-256 master key via Argon2id.
+# Choose a long, random passphrase (e.g., generated by `openssl rand -base64 32`).
+# This must match the passphrase_env setting in mcias.conf.
+MCIAS_MASTER_PASSPHRASE=change-me-to-a-long-random-passphrase