feat(socialcard): add unsplash integration and improve image cards

Add Unsplash API integration documentation to README with setup instructions. Adjust image card gradient scrim to cover bottom 50% with increased opacity (240) for better text readability. Set correct OpenGraph image dimensions (1080x1350) for portrait card formats. Improve Unsplash selection UX with status messages and page reload after successful image download.

README.md

@@ -32,6 +32,7 @@ The AI-native Git platform. Self-hosted, fast, and designed for the age of AI-as
   - [AI Features Configuration](#ai-features-configuration)
   - [Authentication Sources](#authentication-sources)
   - [Email/SMTP Setup](#emailsmtp-setup)
+  - [Unsplash Integration](#unsplash-integration)
 - [Usage](#usage)
   - [Repository Operations](#repository-operations)
   - [Project Management](#project-management)
@@ -549,6 +550,25 @@ USER = your-email@gmail.com
 PASSWD = your-app-password
 ```
 
+### Unsplash Integration
+
+Enable Unsplash image search for repository social card backgrounds (Media Kit). This allows repository admins to search and select high-quality background images directly from Unsplash.
+
+```ini
+[unsplash]
+; Enable Unsplash integration for Media Kit background images
+ENABLED = true
+; Unsplash API access key (get one at https://unsplash.com/developers)
+ACCESS_KEY = your_unsplash_access_key
+```
+
+To obtain an access key:
+1. Create an account at [unsplash.com/developers](https://unsplash.com/developers)
+2. Create a new application
+3. Copy the **Access Key** (the Secret Key is not needed)
+
+When enabled, repository admins can search Unsplash from **Settings > Media Kit** when using the "Background Image" social card style. Attribution is handled automatically per Unsplash API guidelines.
+
 ## Usage
 
 ### Repository Operations