docs: consolidate documentation into organized structure

- Move 12 markdown files from root to docs/ subdirectories
- Organize documentation by category:
  • docs/troubleshooting/ (1 file)  - Technical troubleshooting guides
  • docs/deployment/      (4 files) - Deployment and security documentation
  • docs/guides/          (3 files) - Feature-specific guides
  • docs/planning/        (4 files) - Planning and improvement proposals

Root directory cleanup:
- Reduced from 16 to 4 markdown files in root
- Only essential project files remain:
  • CLAUDE.md (AI instructions)
  • README.md (Main project readme)
  • CLEANUP_PLAN.md (Current cleanup plan)
  • SRC_STRUCTURE_IMPROVEMENTS.md (Structure improvements)

This improves:
✅ Documentation discoverability
✅ Logical organization by purpose
✅ Clean root directory
✅ Better maintainability

src/Domain/SmartLink/Services/ShortCodeGenerator.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Domain\SmartLink\Services;
+
+use App\Domain\SmartLink\Repositories\SmartLinkRepository;
+use App\Domain\SmartLink\ValueObjects\ShortCode;
+
+final readonly class ShortCodeGenerator
+{
+    private const MAX_ATTEMPTS = 10;
+
+    public function __construct(
+        private SmartLinkRepository $repository
+    ) {}
+
+    public function generateUnique(int $length = 6): ShortCode
+    {
+        for ($attempt = 0; $attempt < self::MAX_ATTEMPTS; $attempt++) {
+            $shortCode = ShortCode::generate($length);
+
+            if (!$this->repository->existsShortCode($shortCode)) {
+                return $shortCode;
+            }
+        }
+
+        // If we couldn't generate a unique code after MAX_ATTEMPTS, try with longer length
+        return $this->generateUnique($length + 1);
+    }
+}