[codex] Harden non-system database backup schemas (#4960)

## Thinking Path

> - Paperclip is a control plane whose database is the durable audit and
work record
> - Database backup needs to include operator/plugin schemas while
excluding PostgreSQL-owned internals
> - PostgreSQL reserves the `pg_` schema prefix for system schemas,
including temp and toast variants
> - A single escaped `pg_` prefix predicate is less brittle than
enumerating individual `pg_toast` and `pg_temp` forms
> - This pull request tightens non-system schema discovery for logical
backups without changing the normal user/plugin schema path

## What Changed

- Replaced narrow `pg_toast` and `pg_temp` schema exclusions with an
escaped `pg_` reserved-prefix exclusion.
- Kept `information_schema` excluded from logical backup metadata
discovery.
- Addressed Greptile feedback by removing redundant no-op additions from
the prior iteration.

## Verification

- `pnpm exec vitest run packages/db/src/backup-lib.test.ts`
- PR checks on the latest pushed head: policy, verify, e2e, Greptile
Review, and Snyk

## Risks

- Low risk: PostgreSQL reserves `pg_` schema names for system use, so
this should only exclude database-owned internals that should not be
restored from Paperclip logical backups.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected - check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5 coding agent, tool use and local command
execution. Exact context window was not exposed in the runtime.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

packages/db/src/backup-lib.ts

@@ -239,9 +239,9 @@ function tableKey(schemaName: string, tableName: string): string {
 }
 
 function nonSystemSchemaPredicate(identifier: string): string {
-  return `${identifier} NOT IN ('pg_catalog', 'information_schema')
-    AND ${identifier} NOT LIKE 'pg_toast%'
-    AND ${identifier} NOT LIKE 'pg_temp_%'`;
+  // PostgreSQL reserves pg_ prefixes for system schemas, including temp/toast variants.
+  return `${identifier} <> 'information_schema'
+    AND ${identifier} NOT LIKE 'pg\\_%' ESCAPE '\\'`;
 }
 
 function hasBackupTransforms(opts: RunDatabaseBackupOptions): boolean {