fix(linkedin,pinterest): split CSV/space-joined OAuth scopes before saving

[paulocastellano]

Context

Users connecting LinkedIn (and Pinterest) and posting some time later were hitting:

```
Missing permissions: w_member_social. Please reconnect your account.
```

…even though the LinkedIn account actually had the scope granted, and the daily `social:check-connections` reported the account as healthy.

Root cause

LinkedIn returns the granted `scope` field comma-separated and Pinterest returns it space-separated, but Socialite's scope splitter for each provider doesn't match what they actually return. The net effect: `approvedScopes` is saved as a single-element array containing the entire list as one string:

```
LinkedIn: ['email,openid,profile,r_basicprofile,w_member_social']
Pinterest: ['boards:read boards:write pins:read pins:write user_accounts:read']
```

When `PublishToSocialPlatform` runs:

```php
array_diff(['w_member_social'], ['email,openid,profile,r_basicprofile,w_member_social'])
```

`array_diff` does exact string compare. `'w_member_social'` doesn't equal the CSV string, so it reports it as missing → publish blocked.

Confirmed by inspecting both DB rows and live OAuth response logs.

Fix

Inline re-split at each callback before saving. Provider-specific quirks stay in their own controllers — no shared helper, no centralized abstraction.

`LinkedInController::callback`:
```php
'scopes' => explode(',', implode(',', $socialUser->approvedScopes)),
```

`PinterestController::callback`:
```php
'scopes' => explode(' ', implode(' ', $socialUser->approvedScopes)),
```

The `implode + explode` pattern handles both possible shapes: a single-element CSV (today's bug) or already-split multi-element (future-proof if Socialite ever changes).

Not affected

• TikTok confirmed via debug log: returns 6 properly-split elements. Code unchanged.
• LinkedInPage doesn't write the `scopes` column at all.
• Facebook / Instagram / Threads / YouTube / Mastodon / X / Bluesky: confirmed via DB inspection — all save properly-split arrays.

Existing broken accounts in DB

Will be fixed manually via direct SQL (3 LinkedIn rows + 1 Pinterest row). No migration in this PR.

Test plan

• `php artisan test --compact --parallel` — 1511 passed, 2 skipped, 0 failed (+2 over main)
• New: `linkedin oauth callback splits comma-separated approvedScopes before saving`
• New: `pinterest oauth callback splits space-separated approvedScopes before saving`
• Manual: reconnect a LinkedIn account → DB has 5 separate scope entries
• Manual: reconnect a Pinterest account → DB has 5 separate scope entries
• Manual: attempt to post to a freshly-reconnected LinkedIn account → publish succeeds