fix: resolve memory leak and process crashing during concurrent PDF ingestion

[knoxiboy]

📋 PR Checklist

---

🔗 Related Issue

Closes #565

---

📝 What does this PR do?

Resolves process crashing and memory leaks during concurrent PDF ingestion:

1. Implements concurrency throttling (semaphore) to restrict parallel layout parser and file extraction processing to a maximum of 3 concurrent tasks.
2. Ensures all PDF reader file handles and image-extraction buffers are fully closed and cleaned up in try...finally context managers in AdvancedPDFParser and tasks.
3. Triggers explicit garbage collection (gc.collect()) after layout analysis/extraction operations per page to avoid memory leaks.

---

🗂️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

---

🧪 How was this tested?

• Ran the backend locally
• Tested document uploads manually

---

📸 Screenshots (if UI change)

---

⚠️ Anything to flag for reviewers?

None.

---

✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed