fix(docker): COPY docs/api so api_docs_controller compiles

ApiDocsController + ApiDocsLive read docs/api/{README.md,openapi.yaml}
at compile time and bake the contents into module attributes. The
build context excluded those files because nothing in the Dockerfile
copied the docs directory, so `mix compile` failed during the image
build.

Dockerfile

@@ -66,6 +66,9 @@ COPY algo.md algo.md
 # agent-skills markdown is read at compile time by AgentSkillsController
 # (sha256 digests are baked into the module attribute)
 COPY priv/agent-skills priv/agent-skills
+# /docs/api artifacts are read at compile time by ApiDocsController
+# and ApiDocsLive (baked into module attributes).
+COPY docs/api docs/api
 RUN mix compile
 
 # copy assets and build (JS/CSS changes don't require recompilation)