fix(media): Increase max dimension to 12000px for modern phone cameras

Modern iPhones (48MP) and other phones produce images larger than 4096px.
Since optimize_image() resizes them anyway, the input limit was too
restrictive. Increased from 4096x4096 to 12000x12000.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test_media_upload.py

@@ -127,8 +127,8 @@ class TestImageValidation:
         assert "File too large" in str(exc_info.value)
 
     def test_dimensions_too_large(self):
-        """Test rejection of >4096px image (per ADR-058)"""
-        large_image = create_test_image(5000, 5000, 'PNG')
+        """Test rejection of >12000px image (v1.4.2: increased from 4096)"""
+        large_image = create_test_image(13000, 13000, 'PNG')
 
         with pytest.raises(ValueError) as exc_info:
             validate_image(large_image, 'huge.png')