Add URL generation and signed URL utilities (#16)

* Add URL generation and signed URL utilities

This commit adds utility methods for generating URLs to stored files
and creating cryptographically secure signed URLs for temporary access.

Applications must implement their own serving controllers - this commit
provides tools to make that easier without imposing architectural decisions.

New features:
- FileStorageTable::getUrl() - Generate URLs to serving endpoint
- SignedUrlGenerator::generate() - Create signed URLs with expiration
- SignedUrlGenerator::verify() - Verify signed URLs (timing-safe)
- Comprehensive documentation in docs/Documentation/FileServing.md

Configuration:
```php
// config/app.php
'FileStorage' => [
    'serveRoute' => [
        'controller' => 'Files',
        'action' => 'serve',
    ],
    'signatureSecret' => env('FILE_STORAGE_SECRET'),
],
```

Documentation includes:
- Complete serving controller examples
- 5 authorization patterns (ownership, related entities, roles, etc.)
- Security considerations (hotlinking, rate limiting, etc.)
- Signed URL usage examples
- Testing strategies

Closes #XX

* Fix PHPStan error in SignedUrlGenerator

Cast signature to string for hash_equals to satisfy PHPStan type checking.

* Fix coding standards

* Fix CS

* Remove getUrl() method - let applications handle URL generation

Applications can easily generate URLs using Router::url() directly
in their serving controller context where routing is properly configured.
This reduces unnecessary abstraction and removes the assumption about
routing structure.

Author: dereuromark