feat: forum rich-text editor, emoji picker, mentions, discover nav, feed, uploads, profile

Forum:
- TipTap WYSIWYG editor with full toolbar
- @emoji-mart/react emoji picker (consistent with tweets)
- @mention autocomplete with user search API
- Fix PHP 8.4 parse errors in Blade templates
- Fix thread data display (paginator items)
- Align forum page widths to max-w-5xl

Discover:
- Extract shared _nav.blade.php partial
- Add missing nav links to for-you page
- Add Following link for authenticated users

Feed/Posts:
- Post model, controllers, policies, migrations
- Feed page components (PostComposer, FeedCard, etc)
- Post reactions, comments, saves, reports, sharing
- Scheduled publishing support
- Link preview controller

Profile:
- Profile page components (ProfileHero, ProfileTabs)
- Profile API controller

Uploads:
- Upload wizard enhancements
- Scheduled publish picker
- Studio status bar and readiness checklist

app/Models/PostTarget.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+/**
+ * Polymorphic-style target attached to a Post.
+ * For v1: target_type = 'artwork', target_id = artworks.id
+ */
+class PostTarget extends Model
+{
+    protected $table = 'post_targets';
+
+    public $timestamps = false;
+
+    protected $fillable = [
+        'post_id',
+        'target_type',
+        'target_id',
+    ];
+
+    protected $casts = [
+        'created_at' => 'datetime',
+    ];
+
+    const CREATED_AT = 'created_at';
+    const UPDATED_AT = null;
+
+    public function post(): BelongsTo
+    {
+        return $this->belongsTo(Post::class);
+    }
+
+    /** Resolved Artwork when target_type = 'artwork' */
+    public function artwork(): BelongsTo
+    {
+        return $this->belongsTo(Artwork::class, 'target_id');
+    }
+}