storage: version querylog.db and migrate it in place, never reset a healthy file

querylog.db carries a schema version; migrations run at startup as one transaction after a vacuumed 0600 backup, and every failure refuses startup (exit 2, no systemd restart loop) instead of starting empty. corruption is the only automatic recreate left. the cut gate now requires a fixture-proven migration or an explicit versioned break with restore instructions, and locks shipped migration files and fixtures byte-for-byte.

PLAN.md

@@ -85,13 +85,13 @@ Verified: 0.16.0 ships `std.crypto.tls.Client` only. There is no server-side TLS
 Two SQLite files with opposite write profiles, isolated from each other:
 
 - **`config.db`** — small, precious, rarely written: groups, clients, prefixes, upstreams, blocklist source metadata, rules, local records, forward zones, settings, schema version.
-- **`querylog.db`** — high-churn, large, expendable: query log + its own private `domains` dimension table. Client identity stored as **IP text**, not a FK into config — log rows are immutable facts and must not point at mutable config rows. If `querylog.db` is missing or corrupt at startup, rename aside, recreate, keep serving. Log loss is not an outage.
+- **`querylog.db`** — high-churn, large, expendable: query log + its own private `domains` dimension table. Client identity stored as **IP text**, not a FK into config — log rows are immutable facts and must not point at mutable config rows. If `querylog.db` is missing or corrupt at startup, rename aside, recreate, keep serving — corruption only; a healthy file whose schema this build cannot use refuses the startup instead (§3.7).
 - No cross-DB references. Retention/VACUUM churn never touches `config.db`; config backup is a copy of a tiny file.
 
 ### 3.7 Upgrades: Auto-Migration (Decision J)
 
 - `config.db`: numbered, sequential SQL migration steps compiled into the binary. At startup: read schema version row, apply newer steps inside a transaction, continue. Operator upgrade = install binary, restart. Before v0.1 the list holds one step — the baseline of §11.2, edited in place — because nxdns has no installs and a step exists only to reconcile a database somebody already has.
-- `querylog.db`: **no migrations.** On schema mismatch: rename aside, recreate fresh.
+- `querylog.db`: a logical version in `PRAGMA user_version`, migrated **in place** at startup by the same shape of compiled step list, inside one transaction and behind one `querylog.db.pre-migrate-<epoch>` backup (only the newest is kept). A healthy file is never renamed aside: a version this build cannot reach refuses the startup with instructions, and only corruption recreates. A deliberate break is still allowed, but it must be versioned, refused at startup, and disclosed in the changelog — the cut gate enforces that. See `docs/reference/query-log-lifecycle.md`.
 
 ### 3.8 Blocklist Storage (Decision A)
 
@@ -694,5 +694,5 @@ The project publishes released binaries and container images from its own Gitea
 | G | SQLite vendored amalgamation + own thin wrapper |
 | H | Two DBs: `config.db` (precious) + `querylog.db` (expendable, self-contained, client IP as text) |
 | I | Frontend embedded in binary; dev flag serves from disk; static musl release builds |
-| J | Auto-migration for `config.db` at startup; `querylog.db` recreated on mismatch |
+| J | Auto-migration at startup for both databases; `querylog.db` recreated only when corrupt |
 | — | Safe-search per-group; Prometheus `/metrics` in scope; CI on self-hosted Gitea Actions |