feat(atompub): parse full Org metadata block server-side

[mdorman]

Closes #77

Summary

• parse and validate the leading Org/Jaunder metadata block through one shared normalization interface
• merge web and AtomPub structured metadata with header fallback, then store canonical metadata-free Org source
• validate audiences, lifecycle/date metadata, and identity/sync bookkeeping atomically across SQLite and PostgreSQL
• preserve client compatibility with web, AtomPub, and Emacs round-trip coverage

Verification

• cargo xtask precommit
• cargo xtask prepush
• focused common, storage, web, AtomPub, and Emacs test lanes
• cargo xtask e2e-local posts.spec.ts
• cargo xtask e2e-local atompub.spec.ts
• final two-axis Standards/Spec review and post-rebase conformance review: pass