docs: add .env.example files for client and server

apps/server/.env.example

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+# OpenAI API key for GPT-based chat assistant
+# Required for AI chat functionality
+OPENAI_API_KEY=sk-proj-your-key-here
+
+# Port the server listens on
+# Default: 3000
+PORT=3000
+
+# MongoDB connection URI
+# Default: mongodb://localhost:27017/calchat
+# The Docker Compose setup uses root:mongoose credentials with authSource=admin
+MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
+
+# Use static test responses instead of real GPT calls
+# Values: true | false
+# Default: false
+USE_TEST_RESPONSES=false
+
+# Log level for pino logger
+# Values: debug | info | warn | error | fatal
+# Default: debug (development), info (production)
+LOG_LEVEL=debug
+
+# Node environment
+# Values: development | production
+# development = pretty-printed logs, production = JSON logs
+# Default: development
+NODE_ENV=development