Address code review feedback for RAG agent implementation

[Copilot]

Addresses all feedback from automated code review on the RAG agent implementation.

app.py

• Remove unused asyncio import and metadata variable
• Add proper Content-Type validation via request.is_json
• Add subprocess error checking for crawler/vectorize scripts
• Add input validation: MAX_MESSAGES=50, MAX_CONTENT_LENGTH=100KB
• Replace word-based token estimation with tiktoken

rag_agent.py

• Fix ChromaDB score threshold logic (>= → <= since lower distance = better match)
• Extract duplicate fallback retrieval into _fallback_retrieval() helper
• Add _create_llm_with_temperature() for thread-safe temperature override

# Before: mutating shared LLM state (thread-unsafe)
self.llm.temperature = temperature

# After: create isolated instance
llm_to_use = self._create_llm_with_temperature(temperature)

crawler.py

• Fix page type detection: "latest-news" in page → "latest-news" in self.page.url

vectorize_documents.py

• Use function parameters for chunk_size/chunk_overlap instead of hardcoded values

README.md

• Fix typo "requiments" → "requirements"
• Fix command web.py → app.py
• Fix port references 8000 → 8080

requirements.txt

• Add explicit tiktoken dependency

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