chore: add .env.example for selected use cases

[however-yir]

Summary

Add .env.example files for selected use cases.

Why

A few focused environment templates make it easier to start the most common samples without guessing which Ollama or PostgreSQL settings matter first. This keeps the change small while improving the local setup experience.

Scope

• add .env.example files for chatbot, question-answering, and semantic-search
• keep the templates limited to the most relevant Ollama and PostgreSQL settings
• avoid changing any runtime defaults or build files

Testing

• template-only changes
• reviewed the variable names against the current sample dependencies and README guidance