feat(01-01): create notes table and Note model with polymorphic relationships

- Add notes migration with polymorphic columns (notable_type, notable_id)
- Add foreign key to users table for author tracking
- Add indexes on composite (notable_type, notable_id) and created_at
- Create Note model with morphTo and belongsTo relationships

app/Models/Note.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\MorphTo;
+
+class Note extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'notable_type',
+        'notable_id',
+        'content',
+        'author_user_id',
+    ];
+
+    /**
+     * Get the parent notable model (Member, etc.)
+     */
+    public function notable(): MorphTo
+    {
+        return $this->morphTo();
+    }
+
+    /**
+     * Get the user who authored this note
+     */
+    public function author(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'author_user_id');
+    }
+}