feat(instagram): use custom thumbnail as reel cover via cover_url

[giladresisi]

What kind of change does this PR introduce?

Feature.

Why was this change needed?

Instagram Reels published through Postiz always derived their cover from the video itself (thumb_offset with the captured-frame timestamp). Instagram's Content Publishing API also supports cover_url on REELS media containers, which uses a hosted image as the reel cover in the feed and profile grid. When a reel's media has a thumbnail set, the provider now sends it as cover_url; media without a thumbnail keep publishing via thumb_offset exactly as before. Stories, carousel items and images are untouched. No schema change — this reuses the existing Media.thumbnail column that already flows to providers.

Second commit: the MCP integrationSchedulePostTool now accepts attachments as either a URL string or { path, thumbnail } and forwards thumbnail as the media thumbnail, so MCP clients can set the Reel cover too. The public API already accepted image[].thumbnail, no change needed there.

Docs: gitroomhq/postiz-docs#238, agent docs: gitroomhq/postiz-agent#18 (both to be merged after this PR is deployed).

Other information:

E2e-tested on a Facebook-Login Instagram channel: published reels with both JPEG and PNG cover images and confirmed the cover renders on the profile reels grid (PNG is accepted in practice even though Meta's docs list JPEG only). The standalone Instagram-Login provider delegates to the same postPending, so it is covered by the same code path (not separately e2e-tested for lack of a connected standalone channel).

MCP + public API: published one Reel via integrationSchedulePostTool with { path, thumbnail } and one via POST /public/v1/posts with image[].thumbnail; both covers rendered correctly on Instagram.

Checklist:

• I have read the CONTRIBUTING guide.
• I have signed the Contributor License Agreement (CLA).
• I confirm I have not used AI to submit this PR or generate code for it.
• I checked that there were no similar issues or PRs already open for this.
• This PR fixes just ONE issue

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Strix Security Review

No security issues found.

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[JayChrawnna]

Hi — paying Postiz Cloud customer here. We're scheduling Instagram Reels (standalone IG + share_to_feed) and send a custom cover via image[].thumbnail / Meta cover_url, but Cloud still seems to publish with thumb_offset only, so covers fail silently and we have to set them by hand in the IG app.

Really appreciate this PR. If you can get cover_url into Cloud prod (via staging → prod), it'd unblock us. Happy to test once it's live.

[giladresisi]

Hi, thanks for the detailed report and for offering to test. You're right that Cloud currently publishes Reels with thumb_offset only, so a custom cover passed via image[].thumbnail is not applied yet. This PR adds the cover_url path for exactly that case.

We know it has been sitting open for a while, and we'll try our best to speed things up with our PR review process so it can move through staging to production.