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>

database/migrations/2026_07_27_000001_create_post_social_account_table.php

@@ -0,0 +1,25 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('post_social_account', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('post_id')->constrained()->cascadeOnDelete();
+            $table->foreignId('social_account_id')->constrained()->cascadeOnDelete();
+            $table->timestamps();
+            
+            $table->unique(['post_id', 'social_account_id']);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('post_social_account');
+    }
+};

database/migrations/2026_07_27_000002_migrate_social_account_to_pivot.php

@@ -0,0 +1,33 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        // Copy existing social_account_id values to pivot table
+        DB::table('posts')
+            ->whereNotNull('social_account_id')
+            ->orderBy('id')
+            ->chunk(100, function ($posts) {
+                $now = now();
+                foreach ($posts as $post) {
+                    DB::table('post_social_account')->insert([
+                        'post_id' => $post->id,
+                        'social_account_id' => $post->social_account_id,
+                        'created_at' => $now,
+                        'updated_at' => $now,
+                    ]);
+                }
+            });
+    }
+
+    public function down(): void
+    {
+        // On rollback, we can't easily restore the social_account_id column
+        // since we'll remove it in the next migration. This is fine for a fresh setup.
+    }
+};

database/migrations/2026_07_27_000003_remove_social_account_id_from_posts.php

@@ -0,0 +1,26 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('posts', function (Blueprint $table) {
+            $table->dropForeign(['social_account_id']);
+            $table->dropColumn('social_account_id');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('posts', function (Blueprint $table) {
+            $table->foreignId('social_account_id')
+                ->nullable()
+                ->constrained('social_accounts')
+                ->nullOnDelete();
+        });
+    }
+};