feat(db): copy the database before a boot migrates it

Migrations only go forward, and since #2610 an older image refuses a
database a newer one migrated. Going back after a bad update therefore
needs a copy from before it, and the only copies were backups the admin
takes by hand or auto-backup, which is off by default. The first boot
on 4.4 migrates every 4.3.x install through the legacy baseline and the
new migrations without either.

Before applying pending migrations to a file database that holds
anything, the boot now writes a VACUUM INTO copy next to it, named
pre-migrate-<schema>-<time>.db (legacy-<schema_version> for an install
the positional runner left, otherwise the last recorded migration), and
keeps the newest TREK_DB_PRE_MIGRATE_SNAPSHOT_KEEP (3). The copy is
written under a temporary name and renamed, so a crash never leaves a
truncated file that looks good. If it cannot be written the boot refuses
before anything is migrated; TREK_DB_PRE_MIGRATE_SNAPSHOT=false is the
documented way past that. The newer-database refusal names the copy
taken right before the newer release migrated past this one.

A fresh install, a restart with nothing pending, an in-memory database
and a restore's re-bootstrap (the archive is the copy) take none, and
NODE_ENV=test leaves it off unless a suite sets the switch.

server/.env.example

@@ -48,6 +48,8 @@ DEMO_MODE=false # Demo mode - resets data hourly
 # BACKUP_UPLOAD_LIMIT_MB=500 # Max size (MB) of a backup archive you can upload when restoring. Raise it if your backup exceeds 500 MB. If you sit behind a reverse proxy, raise its upload limit too (e.g. nginx client_max_body_size).
 
 # TREK_DB_JOURNAL_MODE=WAL # SQLite journal mode (DELETE, TRUNCATE, PERSIST, MEMORY, WAL, OFF). WAL is fastest and the right choice on local disk. Set DELETE if the data directory lives on network storage (Azure App Service, an SMB/NFS volume on a NAS) — WAL coordinates writers through a shared-memory file that those filesystems do not implement reliably. The mode is stored inside the database file, so it survives a restart; changing this applies on the next boot.
+# TREK_DB_PRE_MIGRATE_SNAPSHOT=true # Copy the database to data/pre-migrate-<schema>-<time>.db before a startup migrates it, so the previous image can run on the copy again. A copy that cannot be written stops the startup; false migrates without one.
+# TREK_DB_PRE_MIGRATE_SNAPSHOT_KEEP=3 # How many pre-migrate copies to keep, newest first (1 to 100). (default: 3)
 # TREK_DB_SYNCHRONOUS=NORMAL # How hard SQLite flushes to disk (OFF, NORMAL, FULL, EXTRA). Defaults to NORMAL under WAL and FULL under any other journal mode — leave it alone unless you know why you want otherwise. Both variables fall back to their default (with a warning in the log) if the value is not a mode SQLite knows.
 
 # MCP_RATE_LIMIT=300 # Max MCP API requests per user per minute (default: 300)