feat(standard-site): add document coverImage from blog header

[fmalcher]

What

Uploads each blog post's optimized header image as the coverImage blob on its site.standard.document record, so posts carry a thumbnail/cover in the standard.site / AT Protocol network.

How

• DocumentSource gains an optional distDir so the publisher can locate the built, WebP-optimized header at dist/<contentType>/<slug>/<header.url>.
• build.ts passes distDir for the blog source. Material chapters have no headers → source omits distDir, no cover uploaded.
• Per document: read header bytes → uploadBlob → set record.coverImage.
• coverImage is content-addressed, so re-runs with an unchanged image reuse the same CID (idempotent, matches the existing rkey-stable upsert model).

Safety

• Non-fatal per document: a missing, unsupported (e.g. .gif/video header), or oversized (>1MB, per the lexicon guidance) image warns and is skipped — never throws. Publishing already runs inside a non-fatal try/catch with transient-retry.
• Dry-run (STANDARD_SITE_DRY_RUN=true) logs the intended cover upload without calling the PDS.

Verification

• tsc --noEmit clean; full test suite green (262 tests).

Docs updated in STANDARD-SITE.md. Rolls out to both consuming sites via a follow-up submodule bump.