docs: plan backend foundation phase

.planning/PROJECT.md

@@ -54,6 +54,7 @@ each short practice loop.
 - Workflow behavior should be configuration-first and inspired by
   `agent-farm-go`.
 - LLM execution should use `third-one`, defaulting to `deepseek-v4-flash`.
+- The first interview track is Backend Developer Interview.
 - Memory should be structured learner state, not a flat RAG transcript.
 - Gamification should use Flow, adaptive difficulty, growth lines, and strong
   session endings without exploitative mechanics.
@@ -79,6 +80,7 @@ each short practice loop.
 | Structured learner memory, not RAG-first | Product value is learner modeling and readiness | Pending |
 | Game-inspired progression must be evidence-backed | Creates retention without empty rewards | Pending |
 | 600-line source limit | Forces responsibility boundaries early | Pending |
+| Backend Developer Interview first track | Gives a broad but testable MVP concept set | Pending |
 
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-*Last updated: 2026-04-26 after Go backend and GSD planning direction were set.*
+*Last updated: 2026-04-26 after first track and Phase 1 plan were set.*