📝 docs: add two missing ADR files (0011 validation, 0014 gRPC)

ADR 0011 and 0014 were referenced in the README list but their files
were absent from the repository. Reconstruct them from available context:

- 0011: go-playground/validator selection (already implemented in go.mod)
- 0014: gRPC adoption strategy (evaluated and deferred/rejected)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

adr/0011-validation-library-selection.md

@@ -0,0 +1,36 @@
+# 11. Validation Library Selection
+
+* Status: Accepted
+* Deciders: Gabriel Radureau, AI Agent
+* Date: 2026-04-05
+* Implementation Date: 2026-04-05
+
+## Context and Problem Statement
+
+The dance-lessons-coach application needs input validation for API request bodies and configuration values. We need a library that integrates well with Go structs and provides clear error messages.
+
+## Decision Drivers
+
+* Struct-tag-based validation to avoid boilerplate
+* Good error messages with field-level detail
+* Active maintenance and wide adoption
+* Compatibility with existing interface-based design
+
+## Considered Options
+
+* `github.com/go-playground/validator/v10` — struct-tag driven, widely adopted
+* `github.com/asaskevich/govalidator` — tag-based but less expressive
+* Manual validation — full control, no dependency, high boilerplate
+
+## Decision Outcome
+
+Chosen option: **`go-playground/validator/v10`** because it is the de-facto standard in the Go ecosystem, supports struct-tag annotations, provides field-level error detail, and integrates cleanly with our interface-based design.
+
+## Implementation
+
+`github.com/go-playground/validator/v10 v10.30.2` is present in `go.mod`.
+The `pkg/validation/` package wraps the validator for reuse across handlers.
+
+## Links
+
+* [go-playground/validator GitHub](https://github.com/go-playground/validator)

adr/0014-grpc-adoption-strategy.md

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+# 14. gRPC Adoption Strategy
+
+* Status: Rejected / Deferred
+* Deciders: Gabriel Radureau, AI Agent
+* Date: 2026-04-05
+
+## Context and Problem Statement
+
+As the API grows, gRPC was evaluated as an alternative or complement to REST for internal service communication. The question was whether to adopt gRPC alongside the existing Chi REST API.
+
+## Decision Drivers
+
+* Performance of inter-service communication
+* Type safety via Protocol Buffers
+* Streaming support
+* Team familiarity and operational overhead
+
+## Considered Options
+
+* **Hybrid REST/gRPC** — add gRPC endpoints alongside existing REST endpoints
+* **REST only** — maintain current Chi router approach
+* **gRPC-first with transcoding** — use bufbuild/connect for unified REST+gRPC
+
+## Decision Outcome
+
+Chosen option: **REST only (deferred)**. gRPC adoption is not warranted at the current scale. The application has a small number of endpoints, a single-binary deployment model, and no internal service mesh that would benefit from gRPC's efficiency.
+
+### Reasons for deferral
+
+1. **No inter-service communication today** — the application is a single binary; gRPC's main benefit (efficient binary RPC between services) does not apply
+2. **Complexity cost** — adding Protobuf toolchain, code generation, and a second transport layer would significantly increase cognitive overhead
+3. **Chi router commitment** — the REST API is well-designed with OpenAPI documentation; introducing gRPC in parallel creates dual-maintenance burden
+4. **Team capacity** — limited bandwidth for large architectural changes
+
+## When to reconsider
+
+* Application evolves into multiple services that need efficient internal RPC
+* Streaming use cases emerge (real-time lesson progress, etc.)
+* External consumers explicitly require gRPC endpoints
+
+## Links
+
+* [ADR-0002: Chi Router](0002-chi-router.md)
+* [ADR-0013: OpenAPI/Swagger Toolchain](0013-openapi-swagger-toolchain.md)