🔩 Render threaded EML correspondence as polished document pages

[JFWooten4]

Summary

Add first-class, readable, inline EML correspondence pages to the existing read-only WhyDRS document host, with clean filename-derived public slugs and full conversation-thread views.

This is stacked on the interim Nginx document-hosting changes in #8, rather than targeting main directly. Merge that hosting PR first, then this one.

Reader experience

• Adding Board Letter.eml publishes /board-letter/; nested documents retain directory-based URL prefixes, with deterministic disambiguation for filename collisions.
• Each source email gets a permanent, human-readable public URL, but raw EML source files are never served.
• Messages linked by Message-ID, In-Reply-To, and References render as a chronological, inline conversation, with per-message permalinks, a thread navigator, and a consolidated correspondence index at /_mail/. Sibling replies can still group if the original message is missing.
• Responsive layout with sender, recipients, date, subject, and readable HTML or plaintext. Quoted plaintext histories are collapsible.
• Embedded cid: PNG, JPEG, GIF, and WebP images render inline. Other MIME attachments are downloadable with original filenames shown.

Build and security

• A Python standard-library generator produces static HTML and CSS at Docker build time; no runtime backend, frontend framework, JavaScript, or mail-sending system.
• Conservative HTML and CSS allowlists remove active content and network-loaded images; CSP blocks scripts and outbound resource requests. Generated attachments use opaque .bin names with a forced attachment disposition and nosniff.
• Existing Markdown and PDFs remain served unchanged through Nginx, and .dockerignore now admits EML files for conversion without publishing their originals.

Verification

• Five local Python unit tests passed: reply grouping, missing-root sibling threads, HTML sanitization, CID images/attachment output, permalink slugs, collisions, and escaping.
• Local nginx -t syntax validation passed.
• Branch is one commit ahead of serve-documents with seven intended files changed; existing repository documents remain untouched.
• Not verified: full Docker image build or deployment. The repository currently has no .eml document fixtures, so production content rendering still needs an end-to-end check after adding a real EML.

Notes

Thread grouping relies on actual RFC email reference headers, not subject similarity; separate emails that lack reply headers remain separate. HTML style blocks and remote images are deliberately dropped to prevent untrusted email content from controlling the site or tracking visitors.