fix(pinterest): retry transient 'Something went wrong on our end' errors

[giladresisi]

What kind of change does this PR introduce?

Bug fix (backend, Pinterest provider error mapping). Adds a Something went wrong on our end branch to PinterestProvider.handleErrors returning type: 'retry', so SocialAbstract.fetch retries the call up to three times roughly five seconds apart before falling through to a BadBody. Pinterest returns this body for transient failures on their side, which previously fell through handleErrors and failed the post on the first hit as "Unknown Error". The retry budget is the existing one in SocialAbstract.fetch - no new retry mechanism is introduced, and the existing bad-body branches are unchanged.

Why was this change needed?

Pinterest's POST /v5/pins intermittently answers with {"code":2787,"message":"Sorry! Something went wrong on our end."}. This is a transient error on Pinterest's side: identical pins (same board, link, image) go through minutes before and after. Between Aug 12 and Aug 14 we saw a spike of these across many accounts, and two customers reported it.

The response was unmapped in PinterestProvider.handleErrors, so fetch threw a non-retryable BadBody with the Unknown Error placeholder on the first attempt: the post failed for good and the customer only saw "An error occurred while posting on pinterest: Unknown Error".

This maps the message to type: 'retry', so the existing retry path in social.abstract.ts re-attempts up to 3 times (5s apart). If it still fails, the curated message ("Pinterest reported a temporary error on their side. Please try posting again.") is carried into the BadBody, so once #1868 lands it is surfaced in the calendar tooltip instead of the generic text.

Other information:

Same shape as the existing "Unable to reach the URL" retry mapping in the same provider. Not exercised end to end against Pinterest (the error is not reproducible on demand); the retry and message plumbing are the existing paths already used by that mapping.

QA

1. Connect a Pinterest channel and select a board
2. Put a proxy or stub in front of the Pinterest pins endpoint so it returns a body containing "Something went wrong on our end"
3. Publish a pin - backend logs show three attempts roughly five seconds apart before the post fails
4. Change the stub so it returns the error only on the first call - the pin publishes on the retry and exactly one pin appears on the board
5. Remove the stub and publish a pin normally to confirm the happy path is unaffected
6. Confirm a "Board not found" error still fails immediately rather than retrying

Checklist:

• I have read the CONTRIBUTING guide.
• I have signed the Contributor License Agreement (CLA) (ICLA for individuals, CCLA for entities).
• 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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