[codex] Improve Bing metadata image crawlability

[RayLi-Muye]

Refs #2

Scope

• Route Open Graph, Twitter, Organization JSON-LD, and project JSON-LD image/logo URLs through the site-owned /api/media endpoint so crawlers can fetch metadata assets without an OSS page Referer.
• Enable /api/media in production with bucket-relative path validation, a stable https://mist-arch.com/ OSS Referer, and cache headers.
• Update robots.txt to allow /api/media while keeping the broader /api/ path disallowed.
• Add explicit index, follow robots metadata for localized pages.
• Add Playwright coverage for metadata image URLs, the metadata image fetch path, JSON-LD logo URL, and robots rules.
• Allow local Playwright runs to use PLAYWRIGHT_PORT so tests can run without stopping unrelated local services on port 3000.
• Update SEO, image pipeline, and development-log docs.

Verification

• npm run typecheck
• npm run lint
• npm run build
• PLAYWRIGHT_PORT=3005 npm run test:e2e — 9 Chromium tests passed
• git diff --check

Live/diagnostic proof from the audit:

• Current production HTML already has valid title, description, canonical, Open Graph, Twitter, JSON-LD, and non-empty image alt output.
• Direct no-Referer requests to current OSS metadata image URLs return 403 Forbidden, while the same OSS object succeeds when fetched with the production site Referer.
• This PR moves metadata image URLs to a site-owned fetch path; it does not change visible page imagery.

Docs Impact

• docs/SEO.md documents crawler-safe metadata image URLs and the intentional /api/media robots allowance.
• docs/IMAGE_PIPELINE.md documents mediaMetadataUrl() as a metadata-only exception to direct OSS/CDN browser imagery.
• docs/DEVELOPMENT_LOG.md records the change and verification.

Risk / Rollback

• Main risk: /api/media becomes publicly reachable for safe bucket-relative OSS paths. The route rejects absolute/data/empty/traversal paths and sends cache headers to limit repeated fetch load.
• Main benefit: Bing and other crawlers can fetch logo/social preview assets without depending on OSS Referer behavior.
• Rollback: revert this PR to return metadata images to direct OSS URLs and restore /api/media as development-only.

Owner Decision Needs

• Review/merge decision for this PR.
• Separate explicit approval is still required for merge, preview/production deployment, Bing Webmaster manual actions, favicon/logo visual changes, or any customer-visible copy/visual direction changes.