Add Video Projects schema and workflow engine core (tasks #31, #32)

Foundational layer for the YouTube Content Creation System project,
built as a fully independent domain per the earlier decision (task
#47) — no foreign keys into the existing projects/tasks/ideas tables.

Schema: video_projects, workflow_steps, assets, ai_providers,
workflow_templates (named to avoid colliding with the existing
Post-templates `templates` table).

WorkflowEngine (app/Support/VideoWorkflow/) drives a project's step
sequence: start/startFromTemplate, currentStep, completeStep (with
optional approval gating), approveStep/rejectStep, failStep/retryStep,
skipStep. It only manages state transitions — actual step execution
(AI calls, rendering, etc.) is out of scope here (tasks #33+).

Also fixes four Publisher classes (WordPress/Reddit/Twitter/LinkedIn)
that were missing a `use App\Models\SocialAccount;` import after an
earlier interface change, which broke the entire test suite at
bootstrap; and PostFactory, which still set the now-removed
`social_account_id` column instead of the SocialAccount pivot.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

database/migrations/2026_08_06_203013_create_assets_table.php

@@ -0,0 +1,35 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('assets', function (Blueprint $table) {
+            $table->id();
+            $table->foreignId('video_project_id')->constrained()->cascadeOnDelete();
+            $table->foreignId('workflow_step_id')->nullable()->constrained()->nullOnDelete();
+            $table->string('type');
+            $table->string('disk_path');
+            $table->string('mime_type')->nullable();
+            $table->unsignedBigInteger('size_bytes')->nullable();
+            $table->unsignedInteger('version')->default(1);
+            $table->json('metadata')->nullable();
+            $table->timestamps();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('assets');
+    }
+};