feat(api): add block object link REST endpoints with card presentation

[tayuLuc]

Summary

Add REST endpoints for managing object link blocks with full card presentation, matching desktop UI behavior. Links and backlinks now populate correctly when content is created via API.

New endpoints

POST /v1/spaces/{space_id}/objects/{object_id}/blocks/{block_id}/link — Set or update an object link on a block. A text block becomes a link block (card layout); an existing link block gets updated target and presentation. Uses the same internal BlockReplace path as the editor — links/backlinks derive normally.

DELETE /v1/spaces/{space_id}/objects/{object_id}/blocks/{block_id}/link — Remove a link block. Supports optional ?target_object_id= query parameter for safety.

New fields on PATCH …/objects

markdown_append — Append markdown as new blocks at the end without removing existing content. Mutually exclusive with markdown.

New query parameter on GET …/objects

?block_link_candidates=1 — Returns text and link block IDs with metadata (kind, text_preview, target_object_id, link_style, card_style, icon_size, link_description, relations, background_color) so clients can discover which blocks to operate on.

POST …/link request fields

Field	Values
target_object_id	(required) object to link to
link_style	page, dataview, dashboard, archive
card_style	text, card, inline
background_color	grey, yellow, orange, red, pink, purple, blue, ice, teal, lime
icon_size	none, small, medium
link_description	none, added, content
relations	array of relation keys to display on the card
sync_link_presentation_from_block_id	copy icon_size, description, relations, fields from another link block on the same page

Idempotency

When both link payload and block-level chrome (background_color) already match, the endpoint returns replaced: false and skips the BlockReplace call.

Related

• Anytype desktop PR feat: convert anytype:// URLs to Mention marks in markdown parser #2979 (markdown to mention marks) is complementary: this PR covers the programmatic/REST path while that PR covers the import/paste path
• Closes Are block-level REST endpoints planned? (for rich content creation) anytype-mcp#105 (partial — covers block links and card presentation)

Checklist

• New tests for block_object_link service and object service additions
• go build ./core/api/... and go vet ./core/api/... pass
• Generated OpenAPI docs need regeneration via make openapi (swaggo annotations are in place but docs.go, openapi.json, openapi.yaml are not updated in this PR)

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[tayuLuc]

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