Add an ADR directory and backfill the Express-to-NestJS rebuild decision

[james2177]

Description

Git history (git log) shows a full rebuild from Express to NestJS happened (commit history references "Nest rebuild complete"), which is a significant architectural decision, but there's no ADR or written record of why it happened or what alternatives were considered — that context currently only lives in commit messages.

Requirements and context

• This is worth capturing now while the reasoning is still fresh among the team, before it becomes tribal knowledge that's lost

Suggested execution

1. Fork the repo and create a branch

   git checkout -b docs/backfill-nestjs-rebuild-adr
   

2. Implement changes
   Relevant files:

• docs/adr/0000-express-to-nestjs-rebuild.md

3. Test and commit

   • Run npm run lint && npm run typecheck && npm test && npm run test:e2e
   • Include test output / screenshots or logs demonstrating the fix in the PR

   Example commit message:

   docs: backfill ADR for the Express to NestJS rebuild
   

Guidelines

• Assignment required before starting — comment to claim this issue
• PR description must include: Closes #[this issue's number]
• Timeframe: 96 hours from assignment
• Complexity: High (200 points)

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Part of the Drips Wave for vortex-backend.