fix: use redis_url instead of nonexistent redis_host/redis_port in health check

[grod718]

Summary

Fixes the /health endpoint's Redis check, which referenced nonexistent settings.redis_host/settings.redis_port fields, causing it to silently report Redis as unhealthy on every request regardless of Redis's actual status.

Issue

Closes #155

Changes

• Replaced redis.Redis(host=settings.redis_host, port=settings.redis_port, ...) with redis.Redis.from_url(settings.redis_url, ...) in api/routes/health.py, matching the only Redis config field that actually exists on Settings
• Added tests/unit/test_health_routes.py — the first route-level test in this codebase, covering both the healthy and genuinely-unhealthy Redis paths

Testing

• Unit tests pass (make test-unit) — confirmed via git stash comparison that this PR introduces zero new failures; the 53 pre-existing failures in the suite are unrelated to this change (see JOURNAL.md for details)
• Integration tests pass — not run, no integration tests apply to this change
• Linter passes (make lint) — pre-existing ruff warning on the untouched Depends(get_db) default-argument pattern in this file; not introduced by this PR
• Type checker passes (make typecheck) — pre-existing mypy debt across many unrelated files in this codebase (see JOURNAL.md); this PR introduces zero new mypy errors in files it touches
• New/updated tests cover the changes

Screenshots / Demo

curl http://localhost:8000/health before: "redis":"unhealthy" — after: "redis":"healthy" (Redis genuinely healthy the whole time, confirmed via docker compose ps).

Notes for Reviewers

This codebase currently has significant pre-existing test and type-check debt unrelated to this change (documented in JOURNAL.md) — I verified via git stash that none of it is introduced by this PR. Happy to discuss if there's a preferred way to handle the make check/pre-commit gate given this.