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/factories/WorkflowStepFactory.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace Database\Factories;
+
+use App\Models\VideoProject;
+use App\Models\WorkflowStep;
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+class WorkflowStepFactory extends Factory
+{
+    protected $model = WorkflowStep::class;
+
+    public function definition(): array
+    {
+        return [
+            'video_project_id' => VideoProject::factory(),
+            'type' => fake()->randomElement(WorkflowStep::TYPES),
+            'order' => 0,
+            'status' => 'pending',
+            'requires_approval' => false,
+        ];
+    }
+}