feat(rag): add chunking tuning settings and deployment docs (#922)

[AkhileshPachnanda]

Description

Closes #922

This PR introduces configurable RAG document chunk size (rag_chunk_size), allowing users to tune chunking parameters via settings to optimize retrieval quality for different document profiles. It also adds detailed build & deployment documentation on chunking recommendations.

Changes Made

• Backend:
  • Added rag_chunk_size: int = 600 field to AppSettings schema (backend/models/schemas.py).
  • Implemented boundary validation (100–2000 characters) in settings route (backend/routes/settings.py).
  • Updated index_document() in backend/services/rag_service.py to read rag_chunk_size dynamically from settings with fallback to default (600).
• Frontend:
  • Added "RAG Chunk Size" range slider control (100–2000 step 50) to SettingsPanel.jsx.
• Documentation:
  • Added deployment and chunking tuning guide in docs/deployment.md.
• Tests:
  • Updated backend tests (test_api.py) for rag_chunk_size validation & persistence.
  • Updated frontend test mocks in SettingsPanel.test.jsx.

Type of Change

• Feature (non-breaking change which adds functionality)
• Docs (documentation updates)
• Tests (adding or updating tests)

Checklist

• My code follows the project's coding standards.
• All backend unit tests pass (pytest tests/ -v).
• I have updated documentation accordingly.
• My changes generate no new warnings or errors.

How to Verify

Automated Verification

1. Backend Tests:

   cd backend
   pytest tests/test_api.py -v

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