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.

docs/reference/cli.md

@@ -214,7 +214,7 @@ Prints the usage text to stdout and exits 0. `nxdns --help` and `nxdns -h` do th
 | --- | --- |
 | 0 | Success. |
 | 1 | Runtime failure — I/O, database, out of memory. A partial diagnostic report caused by an allocation failure is a runtime failure, not a verdict on the configuration. |
-| 2 | A configuration problem the operator can fix, or a `check` that found one. |
+| 2 | A configuration problem the operator can fix, a `check` that found one, or a deliberate refusal to run that no retry will clear. |
 | 64 | Usage error — unknown command or flag, a flag without its value, a missing or extra argument. |
 
 Code 2 means the same thing from every subcommand. `src/config/faults.zig` holds the one list of errors that mean "the configuration the operator supplied is wrong", and `run`, `check` and `import` all ask it, so a rejected file exits 2 whichever command read it. The list is every error the validator raises, plus `ParseZon`, `ConfigTooLarge`, `NoUsableUpstreams`, `BadCertificate` and `ManagedConfigUnreadable`. In practice that covers a file with a syntax error, one larger than 4 MiB, one with no `default` group (`MissingDefaultGroup`), one with no enabled upstream (`NoUpstreams`), a bad bind address, a bad rate limit, an unusable certificate, `password` and `password_hash` set together, and a `--config` path that is absent or unreadable.
@@ -237,6 +237,8 @@ load one with `nxdns import <file>`, or make a file the source of truth with `nx
 
 `import` exits 2 for those faults and for `DestructiveImport`. That last one is deliberately not a configuration fault — it reports what applying the file would delete, rather than anything wrong with its content — and `import` decides it for itself; the answer to it is `--allow-delete`, not an edit.
 
+`run` also exits 2 on the four query-log schema refusals — `SchemaTooNew`, `SchemaUnsupported`, `MigrationFailed` and `MigrationBackupFailed` — which are in the same list. They are not a verdict on a file the operator wrote, but they share the property exit 2 exists to signal: the server is refusing on purpose, an operator has to act, and a restart will only repeat the refusal. Exit 1 would put them under the unit's `Restart=on-failure` and loop them. See [the server refuses to start over querylog.db](../how-to/troubleshoot.md#the-server-refuses-to-start-over-querylogdb).
+
 `OutOfMemory` is exit 1 even when problems were recorded, because the report is then incomplete. Every other error is 1.
 
 Where an exit code sends you next: [troubleshoot](../how-to/troubleshoot.md).