feat(ops): W4-B single-container packaging, DATA_DIR, backup/restore

- DATA_DIR (/data: agent-sessions, backups, exports, tmp); entrypoint chowns it
  to PUID/PGID then drops privileges via gosu; /app binary stays root read-only
- compile with --allow-read --allow-write so mounted library roots are writable
- preflight: DATA_DIR/session dir must be writable (fatal); enabled library
  roots and downloader path mappings checked read/write (warn + audit)
- --role export-mappings (mapping manifest JSON, --compare row counts) and
  --role read-only-on (automation.readOnlyReconcile=true + audit)
- scripts/backup.sh (pg_dump custom + mappings + agent sessions, SHA256SUMS,
  BACKUP_KEEP rotation) and scripts/restore.sh (checksum, refuses with live
  connections, psql single-transaction replay compatible with PG16, read-only
  reconcile on, row-count verification)
- compose: app + postgres only, data volume, env example with key generation
- docs: runbook deploy.md, backup-restore.md with verification record

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.dockerignore

@@ -14,3 +14,4 @@ docs
 !.env.example
 *.md
 !apps/**/README.md
+data