docs: create architecture decision records (ADRs)

[Agbasimere]

Description

This PR introduces the initial set of Architecture Decision Records (ADRs) for the XStreamRoll codebase. These records document critical design decisions to establish technical context, onboard new team members, and prevent regression or accidental refactoring of intentional patterns.

The new ADRs are located under docs/adr/:

1. 0001-use-nestjs.md: Adoption of NestJS backend framework.
2. 0002-in-memory-first.md: Details the in-memory-first repository pattern.
3. 0003-polling-based-processing.md: Details the polling-based daemon worker processing model.
4. 0004-raw-sql-over-orm.md: Explains using raw SQL queries with pg instead of ORMs.
5. 0005-socket-io-realtime.md: Documents using Socket.IO for real-time events and room management.
6. 0006-npm-workspaces.md: Explains the npm workspaces monorepo structure.

Related Issue

Fixes #232

How to Verify

Verify that all markdown files exist in docs/adr/, read correctly, and conform to standard ADR formats (Title, Status, Context, Decision, Consequences). All files have been successfully validated and formatted with Prettier.

[Xhristin3]

Hey @Agbasimere! These ADRs are excellent - the context-decision-consequences format is spot on and you covered the key architectural choices (NestJS, in-memory-first, polling, raw SQL, Socket.IO, workspaces) really well. Each record clearly explains the why, not just the what. Great documentation work!