Add Cross-Posting support for multiple social accounts

- Create post_social_account pivot table for many-to-many relationship
- Add migration to migrate existing social_account_id to pivot table
- Remove social_account_id column from posts table
- Update Post model with socialAccounts() BelongsToMany relationship
- Update Posts Livewire component to use array of selectedAccountIds
- Update Posts view with checkboxes for selecting multiple accounts
- Update PublishPostJob to iterate through all social accounts of matching type
- Update Publisher interface to accept optional SocialAccount parameter
- Update all Publishers (WordPress, Reddit, Twitter, LinkedIn) to support cross-posting
- Display multiple social accounts as badges in posts table

Now a single post can be published to multiple social accounts of the same type.
For example: a Twitter post can be published to multiple Twitter accounts.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

app/Models/Post.php

@@ -5,6 +5,7 @@ namespace App\Models;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
 
 class Post extends Model
 {
@@ -19,7 +20,6 @@ class Post extends Model
 
     protected $fillable = [
         'site_id',
-        'social_account_id',
         'type',
         'title',
         'body',
@@ -40,8 +40,8 @@ class Post extends Model
         return $this->belongsTo(Site::class);
     }
 
-    public function socialAccount(): BelongsTo
+    public function socialAccounts(): BelongsToMany
     {
-        return $this->belongsTo(SocialAccount::class);
+        return $this->belongsToMany(SocialAccount::class, 'post_social_account');
     }
 }