feat(learning): add roadmap "Cache-aside with Redis" (en/fr)

[OthmaneZ05]

Summary of Changes

Adds the second flagship learning roadmap, "Cache-aside with Redis" (roadmaps/cache-aside-redis.json + French translation cache-aside-redis.fr.json, same id and step ids). Pure content at the frozen v1 roadmap format — zero backend/frontend changes.

The arc (8 steps, ~30 min, sequel to the Nimbus Books story): rebuild a minimal store (web + PostgreSQL, least-privilege security groups from day one) → feel a catalog page that costs ~1500 ms of database time on every request → add a Redis node → implement the cache-aside read path at the terminal (GET → miss → query → SETEX 180 s) → prove the hit with the badge, the Key Explorer and a live TTL countdown → finish on cache invalidation: insert a new book, watch customers get a stale page, validate while stale to read the failure, then DEL the key.

Uses 6 validator types: container_running, table_exists, edge_exists, http_get_contains, redis_key_exists (glob cache:* climax), and a port_denied lock-in check that keeps the cache out of the database. Key/table names (cache:*, books) are chosen not to collide with the explorer demo seeds. Validator order in the cache-aside step is deliberate: the HTTP check's own request causes the first miss that fills the cache, so the key check deterministically fails while the old (non-caching) server is still running.

Types of Changes

• New feature / node type addition
• Bug fix (non-breaking change resolving an issue)
• Refactoring / structural cleanup
• Documentation update

Verification & Testing

Automated Checks

• Run npm run lint successfully with no errors
• Run npm run build successfully with no compilation errors
• Run npm test successfully (all tests pass)

Both files pass npm run roadmap:validate. Backend: 246 tests green. Frontend: 225 tests green.

Manual Verification

Played end-to-end against real Docker containers, twice, with an API driver that mirrors the canvas UI (same network-config/save semantics, prepended SG rules, real docker exec for terminal steps):

• All 8 steps observed fail in the naive state, then pass after the instructed action.
• Counter-tests: validating the cache-aside step with the old server still running fails deterministically on the key check; opening the database to Anywhere flips the port_denied lock-in check red until tightened back to Source=web; the stale-catalog step validated before invalidation fails with served from cache visible in the observed excerpt; stopping web then starting it revives the app via the baked .bashrc boot line; stopping cache leaves the store up (slow, served from database) thanks to the client timeouts + try/except taught in the step.
• Completed steps stay green when re-validated later (e.g. step 4 after the cache exists).
• UI pass (headless Chrome against the dev servers): catalog shows both roadmaps with coherent titles/durations/difficulty; step 1 fail→pass through the player buttons; reopening the roadmap resumes on the first incomplete step with ✓ markers restored; the http://localhost:<port> link on the hit-proving step serves the store from the host; the Redis Key Explorer shows cache:books next to the demo keys and TTL cache:books counts down in the Redis Shell; switching EN→FR mid-roadmap keeps the progression and reopening loads the French file; killing and restarting the backend mid-roadmap resumes on the right step; the finale (stale fail → DEL → pass) completes the roadmap in French.
• Real-traffic spot checks inside the containers: the naive page costs ~1.5 s on every request; the cached page answers in under a millisecond; TTL returns the configured 180 s.

Checklist

• My code follows the repository's code style and lint standards
• I have updated the documentation or instructions if necessary
• All unit and integration tests are passing