Add extensible content parser interface

[pfefferle]

Proposed changes:

• Add a Content_Parser interface for pluggable content parsing of the content union field in site.standard.document records.
• Wire content parsing into the Document transformer with two filters: atmosphere_content_parser (swap/disable parser) and atmosphere_document_content (modify parsed output).
• Add ?atproto preview endpoint on singular posts to inspect the document record JSON (requires edit_posts capability).
• Always set the required site field in document records, falling back to the site URL when no publication record exists.

Split from #3 — this PR contains the generic extensible infrastructure. The Markpub parser implementation follows in a stacked PR.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Verify ?atproto preview on a published post returns valid JSON (when logged in with edit_posts capability).
• Verify ?atproto is not accessible when logged out.
• Verify the content field is absent from the record (no parser registered by default).
• Verify the site field is always present in document records.

Changelog entry

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Changelog Entry Details

Significance

• Patch
• Minor
• Major

Type

• Added - for new features
• Changed - for changes in existing functionality
• Deprecated - for soon-to-be removed features
• Removed - for now removed features
• Fixed - for any bug fixes
• Security - in case of vulnerabilities

Message