BUG: PATCH writes via /api/v1/settings, /admin/themes, /admin/customizer 500 — non-existent namespace column

[tayebmokni]

Summary

Surfaced during work on #499. Three packages share the same write-side
bug — `upsertSQL` writes to a `namespace` column that does not exist
in the `options` table (migration 000008).

Affected files

• `packages/go/settings/postgres.go` — `upsertSQL` and its single caller in `Set`/`SetMany`
• `packages/go/themes/store.go` — same pattern when persisting `active_theme`
• `packages/go/customizer/store.go` — same pattern when persisting overrides

GET path is unaffected; all three select against `key`/`value` only.

Reproduction

1. Boot the stack (`docker compose up -d`).
2. Sign in as admin.
3. Open admin → Settings → General. Change the site name. Save.

Expected: 200, page reloads with new value.
Actual: 500 from `/api/v1/settings` (`column "namespace" does not exist`).

Same reproduces on:

• Appearance → Customize → Save (saves overrides)
• Appearance → Themes → switch active theme

Recommended fix

Either:

(a) Add a `namespace` column to the `options` table via a new migration. Default to `''` for existing rows. Update the GET path's WHERE clauses to filter by namespace too. Riskier — broadens the schema.

(b) Strip the `namespace` column from the three `upsertSQL` statements. The current code never populates it from anything but a hardcoded constant. Cheapest fix; matches the schema as it ships.

Recommend (b) for now. Re-evaluate when multi-tenant work lands.

Acceptance

• Each Settings sub-page "Save" button persists and reloads with the saved value.
• `go test ./packages/go/settings/...` covers a Set→Get round-trip.