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>

app/Models/Asset.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class Asset extends Model
+{
+    use HasFactory;
+
+    public const TYPES = ['script', 'image', 'video', 'audio', 'other'];
+
+    protected $fillable = [
+        'video_project_id',
+        'workflow_step_id',
+        'type',
+        'disk_path',
+        'mime_type',
+        'size_bytes',
+        'version',
+        'metadata',
+    ];
+
+    protected $casts = [
+        'metadata' => 'array',
+    ];
+
+    public function videoProject(): BelongsTo
+    {
+        return $this->belongsTo(VideoProject::class);
+    }
+
+    public function workflowStep(): BelongsTo
+    {
+        return $this->belongsTo(WorkflowStep::class);
+    }
+}