feat: migrate to typed messages, drop base64

- Switch OnMessage → OnTypedMessage with natsutil.Decode[messages.ChatRequest]
- Return *messages.ChatResponse / *messages.ChatStreamChunk (not map[string]any)
- Audio as raw []byte in msgpack (25% wire savings vs base64)
- Remove strVal/boolVal/intVal helpers
- Add .dockerignore, GOAMD64=v3 in Dockerfile
- Update tests for typed structs (9 tests pass)

Dockerfile

@@ -10,7 +10,7 @@ RUN go mod download
 
 COPY . .
 
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-w -s" -o /chat-handler .
+RUN CGO_ENABLED=0 GOOS=linux GOAMD64=v3 go build -ldflags="-w -s" -o /chat-handler .
 
 # Runtime stage
 FROM scratch