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/Support/Publishing/LinkedInPublisher.php

@@ -3,6 +3,7 @@
 namespace App\Support\Publishing;
 
 use App\Models\Post;
+use App\Models\SocialAccount;
 use Illuminate\Support\Facades\Log;
 use Illuminate\Support\Facades\Process;
 

app/Support/Publishing/RedditPublisher.php

@@ -3,6 +3,7 @@
 namespace App\Support\Publishing;
 
 use App\Models\Post;
+use App\Models\SocialAccount;
 use Illuminate\Support\Facades\Log;
 use Illuminate\Support\Facades\Process;
 

app/Support/Publishing/TwitterPublisher.php

@@ -3,6 +3,7 @@
 namespace App\Support\Publishing;
 
 use App\Models\Post;
+use App\Models\SocialAccount;
 use Illuminate\Support\Facades\Log;
 use Illuminate\Support\Facades\Process;
 

app/Support/Publishing/WordPressPublisher.php

@@ -4,6 +4,7 @@ namespace App\Support\Publishing;
 
 use App\Models\Post;
 use App\Models\Site;
+use App\Models\SocialAccount;
 use Illuminate\Http\Client\ConnectionException;
 use Illuminate\Support\Facades\Http;