fix(publish): wire the existing seoTitle field into the document <title>

[asachs01]

Summary

seoTitle is already a fully-built post-type field — declared in buildPostTypeDefaultFields(), seeded by both DB migration paths, editable via the Content Settings panel's SEO field, and even exposed to the token/binding picker ({currentEntry.seoTitle}) and the AI content tool — but the publish and preview render paths never actually consume it.

renderPublishedDataRowTemplate() (server/publish/publicRenderer.ts) and its preview twin handleRowPreview() (server/handlers/cms/data/preview.ts) both set merged.title unconditionally from row.cells.title / draftCells.title. That merged.title is what buildDocumentMetaTags() uses for the <title> tag (src/core/publisher/render.ts), so an author who fills in a distinct SEO title today sees zero effect on the actual <title> output — the field silently does nothing at publish time.

Change

• New resolveEntryDocumentTitle(cells) helper in src/core/data/cells.ts: prefers seoTitle when set (non-empty string), falls back to title, returns null when neither is present (preserving today's exact fallback-to-template behavior for rows with no title at all).
• Both call sites (publicRenderer.ts:182, preview.ts:99) now use this helper instead of the inline title-only check, so publish and preview stay in parity.
• The on-page H1 / {currentEntry.title} binding is completely untouched — traced the actual data flow and confirmed it resolves from publishedDataRowToLoopItem(row)'s spread of row.cells directly into the template's entry-stack frame, never through merged.title. So this change only ever affects the <title> meta tag, never the visible headline.

Test plan

• New test in src/__tests__/server/publicRendering.test.ts: a real postTypes-targeted entry template (not the existing null-early-return fixture) proving seoTitle set → <title> uses it while the H1 binding still renders plain title; seoTitle unset → byte-identical fallback.
• 4 new unit tests for resolveEntryDocumentTitle in src/core/data/__tests__/cells.test.ts.
• Full suite green: 6616 passed / 1 skipped / 0 failed (bun test).
• tsc -b clean, eslint . clean.

Noticed while working on the trusted-host iframe-embed fix (#369) — same investigation trail, unrelated bug though, so opened as a separate PR for independent review.