Add Platform::altTextMaxLength() as the alt-text cap source of truth

app/Enums/SocialAccount/Platform.php

@@ -127,6 +127,33 @@ public function maxImages(): int
         };
     }
 
+    /**
+     * Character cap the platform's API accepts for image alt text (accessibility
+     * description), or null when the platform has no alt-text field. Facebook and
+     * Threads document no limit, so a defensive 1000 is used. Single source of
+     * truth — publishers truncate to this value, never a literal.
+     */
+    public function altTextMaxLength(): ?int
+    {
+        return match ($this) {
+            self::Bluesky => 2000,
+            self::X => 1000,
+            self::Mastodon => 1500,
+            self::LinkedIn, self::LinkedInPage => 4086,
+            self::Facebook => 1000,
+            self::Instagram, self::InstagramFacebook => 1000,
+            self::Threads => 1000,
+            self::Pinterest => 500,
+            self::Discord => 1024,
+            self::TikTok, self::YouTube, self::Telegram => null,
+        };
+    }
+
+    public function supportsAltText(): bool
+    {
+        return $this->altTextMaxLength() !== null;
+    }
+
     /**
      * Hard cap (in characters) the platform's API will accept. Going over this
      * means the post can't be published. Values are the documented API maxes:

tests/Unit/PlatformAltTextTest.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+use App\Enums\SocialAccount\Platform;
+
+test('altTextMaxLength returns the documented cap for supporting platforms', function () {
+    expect(Platform::Bluesky->altTextMaxLength())->toBe(2000)
+        ->and(Platform::X->altTextMaxLength())->toBe(1000)
+        ->and(Platform::Mastodon->altTextMaxLength())->toBe(1500)
+        ->and(Platform::LinkedIn->altTextMaxLength())->toBe(4086)
+        ->and(Platform::Instagram->altTextMaxLength())->toBe(1000)
+        ->and(Platform::Pinterest->altTextMaxLength())->toBe(500)
+        ->and(Platform::Discord->altTextMaxLength())->toBe(1024)
+        ->and(Platform::Facebook->altTextMaxLength())->toBe(1000)
+        ->and(Platform::Threads->altTextMaxLength())->toBe(1000);
+});
+
+test('altTextMaxLength is null for platforms without alt-text support', function () {
+    expect(Platform::TikTok->altTextMaxLength())->toBeNull()
+        ->and(Platform::Telegram->altTextMaxLength())->toBeNull()
+        ->and(Platform::YouTube->altTextMaxLength())->toBeNull();
+});
+
+test('supportsAltText mirrors altTextMaxLength', function () {
+    expect(Platform::Bluesky->supportsAltText())->toBeTrue()
+        ->and(Platform::TikTok->supportsAltText())->toBeFalse();
+});