docs: plan backend foundation phase

docs/planning/INTERVIEW_TRACKS.md

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+# Interview Tracks
+
+## MVP Track: Backend Developer Interview
+
+The first interview track is backend developer preparation. This track is broad
+enough to be valuable to job seekers and narrow enough to produce testable
+diagnostic, grading, memory, and progression behavior.
+
+## Target User
+
+- Junior backend developer candidates.
+- Bootcamp graduates preparing for backend interviews.
+- Full-stack developers who need backend interview readiness.
+- Developers changing stack into backend service work.
+
+## Canonical Concept Seed List
+
+### 1. HTTP and Web Fundamentals
+
+- request and response lifecycle
+- headers, status codes, methods
+- idempotency and safe methods
+- cookies, sessions, tokens
+- latency, timeout, retry basics
+
+### 2. REST and API Design
+
+- resource modeling
+- endpoint shape
+- validation and error responses
+- pagination and filtering
+- versioning
+- backward compatibility
+
+### 3. Databases and Indexes
+
+- relational modeling
+- primary keys and foreign keys
+- index purpose and tradeoffs
+- query plans at a conceptual level
+- N+1 query problem
+- migrations
+
+### 4. Transactions and Consistency
+
+- ACID
+- isolation anomalies
+- transaction boundaries
+- optimistic vs pessimistic locking
+- idempotent writes
+- eventual consistency basics
+
+### 5. Caching
+
+- cache-aside
+- TTL and invalidation
+- stale data risks
+- cache stampede
+- CDN vs application cache
+- when not to cache
+
+### 6. Concurrency and Async Work
+
+- race conditions
+- locks and coordination
+- worker queues
+- retry and dead-letter basics
+- backpressure
+- Go goroutine/channel concepts for Go-oriented tracks
+
+### 7. Testing and Reliability
+
+- unit vs integration tests
+- test doubles
+- contract tests
+- observability basics
+- logging and metrics
+- failure-mode thinking
+
+### 8. System Design Basics
+
+- load balancing
+- horizontal scaling
+- stateless service design
+- database bottlenecks
+- rate limiting
+- file/object storage
+- basic architecture tradeoffs
+
+## Challenge Ladder
+
+Each concept should support five levels:
+
+1. Define the concept.
+2. Explain tradeoffs.
+3. Debug a realistic scenario.
+4. Design under constraints.
+5. Answer under interview pressure.
+
+## First Boss Question
+
+"Design a rate-limited backend API for submitting job applications. Cover HTTP
+API design, database transaction boundaries, duplicate submission prevention,
+cache behavior, failure handling, and test strategy."
+
+## MVP Exclusions
+
+- Advanced distributed systems theory.
+- Deep database internals.
+- Language-specific trivia-heavy questions.
+- Company-specific interview packs.
+- Frontend, mobile, AI, and DevOps tracks.