docs: plan backend foundation phase

docs/planning/PRD.md

@@ -85,12 +85,13 @@ The first MVP should prove one loop:
 7. User sees visible readiness progress, next unlocks, and a recommended next
    challenge.
 
-Recommended first track:
+First track:
 
 - Backend developer interview preparation.
-- Topics: HTTP, REST, databases, transactions, caching, concurrency, testing,
-  system design basics, Go or JavaScript/TypeScript depending on first content
-  corpus.
+- Topics: HTTP and web fundamentals, REST/API design, databases and indexes,
+  transactions and consistency, caching, concurrency/async work, testing and
+  reliability, and system design basics.
+- Canonical seed list: `docs/planning/INTERVIEW_TRACKS.md`.
 
 ## Core User Flows
 
@@ -183,6 +184,8 @@ the configured image generation provider.
   configuration.
 - Workflow outputs SHALL prefer typed JSON contracts for grading, memory
   extraction, ontology updates, and review-plan generation.
+- Initial workflow contract plan SHALL follow
+  `docs/planning/WORKFLOW_CONTRACTS.md`.
 
 ### Visual teaching assets