feat: landing pages + URL redirects + CMS root-level routing

[treoden]

Three interlocking bodies of work.

URL-redirect subsystem

• New url_redirect table (base) with write-time chain collapse; a notFound-gated middleware 302s an old path to the new one.
• Inline capture in the product/category update services: url_key renames, category reassignment/unassignment (incl. whole variant groups), and category rename AND reparent.
• Unified, unit-tested path algebra (pathRemap: buildEntityPath / remapPath / planSubtreeRedirects); boundary-anchored subscriber cascade (fixes the /shoe-sale sweep and /cat/cat-toy mangle); set-based recordRedirectsBatch.
• Delete-cascade cleanup: purge a category subtree's aliases and capture the nested->root move of every product it uncategorises.

CMS root-level routing

• CMS pages are served at root-level /<url_key> via url_rewrite; the page-builder for CMS pages reverts to route-level.

Landing pages (promotion module)

• New first-class entity: root-level URL, page-builder-only body via entity_urn, admin grid + CRUD + duplicate, status + publish schedule; wins URL collisions.
• Page-builder entity-scope registry; draft/scheduled pages preview in the editor via a valid changeset token (possession-based trust).

SEO / UI

• LinkPicker "Landing page" tab + a promotion:landing_page link loader.
• Live SearchSnippetPreview (SERP snippet) on the landing-page and CMS-page SEO forms; dropped meta_keywords from landing pages; two-column landing edit layout with status + schedule in the sidebar.

Verified: tsc 0 errors; jest 82 suites / 790 tests green.

PR Checklist

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• Tests for the changes have been added (for bug fixes / features)

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What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
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What is the current behavior?

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What is the new behavior?

Does this PR introduce a breaking change?

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