[P5] Extension pack v1: sqlalchemy, redis, sentry, auth (parity with CNA addons)

[ulises-jeremias]

Problem

CPA ships 4 extensions (docker, docker-compose, postgres, vscode) vs CNA’s 51. After layered CI (#46/#47), the catalog is trustworthy but thin. Growth needs a curated extension pack with L2 coverage, not a random dump.

Proposed pack (v1)

Extension	Purpose	L2 template
sqlalchemy / alembic	ORM + migrations for FastAPI	fastapi-starter
redis	Cache/queue client	fastapi-starter
sentry	Error tracking	both starters
auth-jwt or auth-session	Auth skeleton	fastapi-starter

Optional stretch: celery as extension if celery-worker template (#37) is deferred.

Acceptance criteria

• Each extension has cpa.extension.json + package/ + slot hooks
• Registered in extensions/index.json with correct category (database / ci / etc.)
• L2 matrix includes one cell per new extension × canonical FastAPI template
• Docs: docs/ADDING_EXTENSIONS.md examples updated if patterns change
• No regression on L0–L3 green on main

References

• Open: [P5] Additional templates: cli-starter, celery-worker (phased) #37 (templates)
• Done: ci: replace stacked/random extension matrix with trustable layered CI (uvx from PyPI) #46 layered CI
• CNA: cna-templates/extensions/*
• Maturity plan Tier 5 / item 20