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/main.go

@@ -40,7 +40,7 @@
 //	pgcreds get --id UUID
 //	pgcreds set --id UUID --host H --port P --db D --user U
 //
-//	rekey
+//	snapshot [--retain-days N]
 package main
 
 import (
@@ -68,6 +68,14 @@ func main() {
 	command := args[0]
 	subArgs := args[1:]
 
+	// snapshot loads only the config (no master key needed — VACUUM INTO does
+	// not access encrypted columns) and must be handled before openDB, which
+	// requires the master key passphrase env var.
+	if command == "snapshot" {
+		runSnapshot(*configPath, subArgs)
+		return
+	}
+
 	// schema subcommands manage migrations themselves and must not trigger
 	// auto-migration on open (a dirty database would prevent the tool from
 	// opening at all, blocking recovery operations like "schema force").
@@ -273,6 +281,11 @@ Commands:
   rekey                  Re-encrypt all secrets under a new master passphrase
                          (prompts interactively; requires server to be stopped)
 
+  snapshot               Write a timestamped VACUUM INTO backup to
+                         <db-dir>/backups/; prune backups older than
+                         --retain-days days (default 30, 0 = keep all).
+                         Does not require the master key passphrase.
+
 NOTE: mciasdb bypasses the mciassrv API and operates directly on the SQLite
 file. Use it only when the server is unavailable or for break-glass recovery.
 All write operations are recorded in the audit log.