deps: upgrade node 22 → 26 (Docker base image major)

[JasonTM17]

Dependabot major bump: backend/Dockerfile FROM node:22-alpine → node:26-alpine. Plus @types/node 22 → 25 in backend devDeps.

Risk

• Node 26 may not be LTS yet — verify before adopting
• ESM behavior, perf characteristics differ
• Compatibility with all dependencies (Prisma, BullMQ, Nest)

Action items

• Verify Node 26 LTS status
• Test backend image build + startup on Node 26
• Check distroless variant exists for Node 26
• Bump in single PR after green smoke test

Refs

• ADR slot: docs/adr/0008-node-26-upgrade.md (TBD)