Quick how-to / action

• Q1: How do I undo my last git commit but keep the changes?
• Q2: How do I find which process is using port 3000 on Linux and kill it?
• Q3: How do I add an environment variable to a Next.js app and read it in browser code?
• Q4: How do I create a Python virtual environment and install the requests library into it?

Explanation

• E1: Explain how HTTP caching with ETag and Cache-Control works.
• E2: What is the difference between a process and a thread?
• E3: Explain what a database index is and when it can hurt performance.

Detail-critical

• D1 (architecture trade-off): We are a 6-person team building a B2B SaaS with about 200 customers, and we expect 10x growth in two years. Should we start with a monolith or microservices? Give me the trade-offs.
• D2 (security caution): I want users to upload profile pictures to my Express app, store them on disk under /uploads and serve that folder statically. Is there anything I should be careful about?
• D3 (multi-step migration): I need to rename the column users.email to users.contact_email in a production Postgres database while the app keeps running with zero downtime. Walk me through the migration.