fix multiupload and control incomplete pairs

[ElGranLoky]

Description

Upgrade the certificate upload system from handling single files to supporting multiple simultaneous uploads with built-in validation for certificate/key pairs.

Key Changes:

• Support for Multiple Uploads: The /api/upload endpoint was updated to use upload.array('certificates'), allowing users to upload several files at once.
• Automatic File Grouping: Added logic to group files by their base name. For example, if you upload domain.crt and domain.key, the system recognizes they belong together.
• Type Identification: Files are now automatically categorized as cert, key, or p12 based on their extensions (.pem, .crt, .key, .p12, etc.).
• Robust File Handling: Replaced simple file moving with a "copy and unlink" strategy. This prevents the EXDEV error that often occurs when moving files between different disk partitions (like from a /tmp folder to the storage directory).
• Structured Results: The API now returns a detailed report instead of a simple success message, including counts of complete pairs, lists of incomplete pairs, and a summary of all files processed.

---

What This Solves

• Incomplete Certificate Sets: It solves the issue of "orphaned" certificates. The system now detects if a certificate is missing its corresponding private key (or vice-versa) and alerts the user through the incompletePairs list.
• Manual Effort: Users no longer need to upload files one by one; they can drag and drop all components of a certificate (and even multiple different certificates) in one go.
• System Reliability: By ensuring the upload directory exists and using a more compatible file-moving method, the application is less likely to fail due to filesystem permissions or storage configurations.
• Better Client Feedback: The frontend now receives precise information about what was saved successfully and what components are still missing to make a certificate functional.