Add Bluesky Metadata Fields to Post Editor

[poppastring]

Enhance the post editor to include Bluesky-specific metadata such as:

• Bluesky post URI
• Bluesky thread URI
• Custom Bluesky summary
• Custom OG image for Bluesky
• Toggle for "Publish to Bluesky"

Proposed Solution

• Add a Bluesky section to the post editor UI.
• Store metadata in the post XML/JSON.
• Use metadata during publish/update operations.

Technical Notes

• Metadata should not break existing post formats.
• Provide defaults when fields are empty.

Acceptance Criteria

• Authors can configure Bluesky metadata per post.
• Metadata is saved and used during publish.
• UI is clean and optional.